Best Filter Choices for Goldfish Tanks - A Complete Guide to Making Sure The Water In Your Aquarium Stays Clean and Clear

by Adam Till

Short Take

Let our little goldfish paste give you a quick intro...

One of the realities of keeping a goldfish tank is that the waste they produce stays with them in the tank and needs to be dealt with. In nature the flowing current of a river would wash it away or a lake would dilute it, but even a large aquarium contains nothing close to the volume that would be found in the wild.

As a result, at home you need to turn to aquarium filters in order to help reduce goldfish waste to other substances which are (relatively) harmless. Healthy filters provide a perfect place for bacteria to do that job for you, and allow your goldfish to live a long and happy life in your home.

If you're in a rush and looking for a quick filter recommendation for your tank, I'd suggest running a sponge filter powered by a reliable air pump:

  1. sponge filter (use one of these per 20 gallons) and a
  2. Fluval Q1 air pump

That's basically what I run in all our goldfish tanks from 5 gallons to 125 gallons, and with the larger tanks I simply use multiple sponge filters instead of just one. They're cheap, reliable, easy to clean, won't snag long fins, and last forever.

If you can't help but want a power filter instead, your best choices will be:

  1. any of the AquaClear hang-on-back style filters, sized one or more up from what the box says for your tank (ie, use an Aquaclear 30 or 50 on a 20 gallon tank) or
  2. the not-really-required-but-high-tech Fluval FX-series canister filters, of which the FX6 is king. These fancy filters are nowhere near necessary, but every goldfish keeper should own one at least once to see if they like the concept of a canister filter (they're the current "cool" choice)

Quick Navigation

How To Not Overcomplicate Choosing A Goldfish Filter

If you head over to Amazon or look through the filter section of any decently stocked aquarium store, you might get a bit overwhelmed by the sheer number of filters that are out there. There are dozens of different styles and hundreds of models available from a host of different companies.

Before you give up and take up some other hobby instead, however, it's worth taking a second to realize that they all do exactly the same thing in the end - clean the ammonia and debris out of the water, reduce them down to nitrate, and create enough circulation in the aquarium so that there aren't any "dead zones" of water that aren't moving. Everything else outside of that is usually just a pretty box picture, creative marketing mumbo-jumbo, and maybe a guilt trip or two.

Sales pitches will tell you all about the features of one filter over the other, and how spending hundreds of dollars on this fancy filter will yield amazing benefits ( increased water clarity, longer fish life...even bragging rights at the local fish club). What they don't tell you, however, is that the same results can be obtained from far simpler and less expensive filters as well. A BMW might have more bells and whistles than a Honda and appeal more to certain people, but in the end both hopefully get you from A to B.

If you already have a filter now and when you test the water in your mature (>3 month old) aquarium and you read zero ammonia, zero nitrite, and some amount of nitrate, you DO NOT need to update your filter.

Doing so might be fun and educational (nothing wrong with that), but your goldfish won't be healthier if you do. You might WANT a newer, jazzier filter, but you don't NEED it.

What Does A Goldfish Tank Filter Actually Do?

A filter has only a handful of jobs. Those include:

  1. providing ample room for bacteria to grow
  2. making sure that the water in the aquarium circulates enough so that there are no dead zones
  3. (optionally) providing some form of "mechanical" filtration ( to remove suspended debris from the water that make it look cloudy)
  4. (optionally) providing area where some "chemical" filtration material can be added (like activated carbon to remove medication or Purigen to remove the tannins that driftwood adds that make water tinted yellow)
  5. (optionally) using features like UV sterilizers to help kill pathogens or algaes suspended in the water

That's basically it.

As a result, if there's a "secret" nobody else is talking about when it comes to filtration, it's that fundamentally a filter is just a box that contains a lot of area for bacteria to grow combined with a fine net to catch debris.

One common misconception, for example, is that the filter itself actually affects the water directly. Other than removing debris in the same way you could with a fine net, the filter doesn't really do anything on its own.

A newly purchased and installed filter needs time to build up a healthy population of bacteria, and the tank water needs to be monitored and tested regularly while this process (called "cycling") is occurring.

A lack of filter bacteria is why fish will still die in a new tank even with the best filter in the world if the water isn't monitored and changed regularly while it's establishing (and even afterwards to remove end-product nitrates).

A Quick Refresher On "Cycling" An Aquarium

When you set up a brand new tank with a brand new filter, all you've done is create an environment that's perfect for filter bacteria to grow in. Those bacteria are commonly found in the air and the water that comes into our homes, but they aren't initially found in concentrations that would support a goldfish.

Once there is ammonia in the aquarium (either added by you as part of a fishless cycle or added by a new fish by their normal biological processes), the first group of bacteria start to grow and multiply. This doesn't happen without ammonia in the tank, so remember that just filling a tank with water doesn't actually do much of anything.

Those nitrosomonas bacteria (you can forget that term now) convert very toxic ammonia to less toxic nitrite, and as they multiply in number the ammonia reading on your water test kit will start to drop.

If you don't initially change enough water to keep ammonia levels low before your filter bacteria are able to do the job, your fish will die. You may have come across the term "new tank syndrome"; that's just a fancy, medical-sounding term that means that the fish died due to dirty water and ammonia poisoning.

Shortly after the ammonia levels start to drop, nitrobacter bacteria start to grow. These bacteria convert the nitrite left after ammonia is converted into nitrate, which isn't quite harmless but isn't toxic either until larger concentrations are reached.

When both of those bacteria colonies are healthy and happy, and ammonia and nitrite levels are both 0 with the aquarium processing the same amount of ammonia that your goldfish will produce later, your fish has a home that it can safely live in. At that point, we call an aquarium "cycled".

What Happens If the Tank Seems Cycled Before Adding Fish, but I Read Ammonia After the Fish Get Added?

If you're fishless cycling using something like fish food or waste food, you'll likely be able to follow the tank through the cycling process and watch the ammonia and nitrite levels slowly drop to 0.

If you get ammonia or nitrite spikes after you add the fish, you might be confused as to whether the tank is actually cycled, or whether you did something wrong.

The issue then comes from the fact that there just aren't ENOUGH bacteria in the filter to support the bigger fish load. After all, logically a small amount of ammonia and nitrite requires a smaller number of bacteria than a larger one.

Being a bit more patient and changing water as you need to by your test kit will quickly get things under control. Once there's some amount of healthy bacteria present, getting the colonies to expand takes a lot shorter a period of time than establishing them in the first place.

This is why I generally recommend "fish in" cycling when establishing a new tank. A lot of the internet seems to think this method is "cruel", but it only need be if waste levels are allowed to build up without being monitored and reduced (by water changes). 

With fish in cycling the process tends to even go quicker, since bacteria the fish carry in in their wastes tends to kick start the process.

Where most people go wrong with fish in cycling is wanting to leap right to feeding the fish 3 meals a day plus snacks. The waste that results will quickly turn an unycled tank toxic, and the fish will die unless a lot of water is changed frequently.

A better method that merely requires some self control is to feed the fish far less frequently. That might look like the following:

  • Week 1: feed on Monday once
  • Week 2: feed on Monday and Wednesday once
  • Week 3: feed Monday, Wednesday and Saturday once
  • Week 4 Feed Monday, Wednesday, Friday and Saturday
  • Week 5 Feed Monday, Tuesday, Wednesday, Friday and Saturday
  • Week 6 Feed Monday, Tuesday, Wednesday, Thursday, Friday and Saturday
  • Week 7: feed once a day, 7 days a week
  • Weeks 8-16, add a second feeding once per day according to the above schedule
  • Weeks 17-24, add a third feeding once per day according to the above schedule

This method isn't something a lot of new goldfish owners will immediately feel comfortable for fear of starving their fish, but rest assured that they're just fine doing so, and will appreciate less stress from nasty, unprocessed waste products.

Feeding like this will avoid having to do almost daily water changes as can happen if feeding patterns start strong out of the gate, but periodic checking of waste levels is still a good idea.

What Happens to The Nitrate In A Goldfish Tank Though?

That's a good question, and from the perspective of choosing a filter, nothing happens to the nitrate 99.9% of the time.

Except in very rare, usual cases, the only way to remove nitrate from an aquarium is to change water or use something like plants or chemicals to break down the nitrate. The filter itself, however, can't really do anything to the nitrate levels in your tank.

This is why there's no point in running a hugely oversized filter or multiple fancy filters on a tank in the hope that you'll have to do less maintenance (as so many people seem to want to).

Those filters will have more room to capture floating debris and therefore won't require cleaning as often, but the aquarium will still require the same number of water changes as one that's running only a single, appropriately sized filter.

As a result, when your water test kit is showing that you're getting to the 40ppm nitrate level, you have to do a water change. Getting a "better" filter (or adding multiple filters) normally won't do anything to reduce the number of water changes required.

Can I Jump Start the Cycling Process For My Goldfish Tank With One Of the New "Bacteria In A Bottle" Products?


Bacteria in a bottle products like the ones listed below are among a new wave of bottled products promising to "instant cycle" your aquarium.

API Quick Start, Fluval Cycle, and Shrimps Forever Pro Bacter are among the new products which promise live bacteria in a bottle to help "quick start" the filter of your aquarium

While some do seem to help the job get done quicker, a lot depends on both how old the product is and how it was treated both in transit to the store and at the store itself.

Is it old? Did it freeze in the truck on the way there?

All those and questions and more will have an effect on how many live bacteria still remain in the bottle when you go to use it. After all, the products are claiming to contain LIVE bacteria, and all live organisms have their tolerances.

As a result, if you do choose to try one of these products make sure you still treat the resulting aquarium (at least at first) as if it isn't cycled. Don't immediately stock it full of fish you feed 3 times a day, and don't skimp on testing your water.

Are Goldfish Really "Messy" or "Dirty" Fish To Keep?

If you're a fishkeeper who's had nothing but neon tetras, bettas and corydoras in previous tanks (small community fish), then yes, a goldfish might seem like a shock to the system when it comes to their maintenance requirements.

Even keepers of "bigger" fish like angelfish might be surprised at the amount of waste that a 6" goldfish puts out compared to that of a 6" angelfish.

Keepers of Oscars, African cichlids and "monster" (huge) fish likely won't notice anything usual in terms of care with goldfish, however. In fact, they may even find that a goldfish tank needs LESS maintenance than that of one of those other sorts of fish.

The question of whether goldfish are actually "really dirty" is thus one of perspective.

Ounce for ounce, I personally don't believe goldfish create any more mess than any other type of fish.

In fact given that goldfish will go back and "re-digest" waste they've already expelled (poop-eating for the win!), they actually end up making more use out of every ounce of food fed to them than a lot of carnivorous fish like Oscars (who don't often bother hunting down the scraps of what they're fed).

Likewise, the reason that the big angelfish seems like a lighter bioload is largely because the thin profile of an angelfish means that they're very likely to be a third or less of the mass of a goldfish of compatible "visual" size. Pound-for-pound they may make as much waste, but the angelfish just doesn't weigh as many pounds for the same visual size.

As a result, although you should certainly respect the waste output potential of goldfish and choose a filter that can handle the job, don't necessarily buy into the hype of goldfish being "dirty" on any especially terrible level. Choose a filter that's up to the job, but don't worry about getting carried away mortgaging the house for one.

How Do I Know If I Need A New Goldfish Filter?

If you're wondering if your aquarium could benefit from a new filter, ask yourself the following questions (feel free to click on a question that catches your eye):

  1. does it have a filter at all?
  2. does your water measure at 0 ammonia and 0 nitrite after it's been cycling for a few months?
  3. is your filter noisy?
  4. does your filter contain cartridges that are costing you a lot of money each month?
  5. does maintaining your filter take too much time, or make too much mess?
  6. does your filter use too much power?
  7. is your water clear enough for your preferences?
  8. are there any "dead spots" in your aquarium where the water isn't moving?
  9. do you just want a change?

Does Your Goldfish Tank Have A Filter At All?

If your goldfish tank doesn't have a filter at all, you really need to be an advanced hobbyist (and usually one that loves huge water changes) to keep goldfish alive and thriving this way.

As a result, if you don't have a filter at all, please buy one. If you're just starting out, I really recommend these Aquaclear filters (fully filled with foam) as your first filter:

The AquaClear line of filters is a perfect first filter. Ideally buy one a size or two bigger than recommended for your tank (bigger allows for slightly less frequent maintenance and greater capacity)

If you're interested in methods which allow goldfish to be kept without filters, here's a link to a primer on one method. 

Just be aware that these very traditional Japanese methods are a far cry from a tiny unfiltered goldfish bowl and require a huge amount of experience to work successfully.

Does Your Water Measure At 0 Ammonia And 0 Nitrite After It's Been Cycling For a Few Months?

An aquarium that consistently shows a reading for ammonia and nitrite AFTER it's been running for at least a couple of months may not have quite enough media capacity to support the amount of fish in the aquarium. 

As a result, if you've been keeping up on your normal maintenance (periodic gravel vacuuming, washing the filter media gently in tank water during water changes) and you still can't get to the magic 0 ammonia and 0 nitrite levels, you might need to up-size your filter or add another one.

If you're getting 0 ammonia and 0 nitrite but are having trouble keeping nitrate levels below the safe zone of 40 ppm, I'm afraid that won't be solved by adding a new filter.

Since in 99.9% of aquariums nitrate is the "end product" of the filter cycle, the only short-term and sustainable way to remove this is with water changes.

Up-sizing to a larger tank will reduce the time it takes to get to toxic levels of nitrate. Since doing a water change on a large tank isn't really all that much more work than doing one on a small tank after all the equipment is taken out and put away, swapping out a small tank for a larger one is what most people do when they're having trouble keeping up on nitrate maintenance.

If you just can't afford a new aquarium or don't have the space for one, the harder choice is to reduce the number of fish or to choose a different type of fish. Goldfish keeping isn't the easiest hobby in the world, and high nitrate levels will eventually cause your fish health problems in the end.

Planted goldfish tanks are an option for some people to help reduce nitrate levels long term, but just be aware that it takes a large amount of plant mass to balance out the needs of even a small number of goldfish.

Lastly, chemical nitrate removing pads are used by some to offset nitrate buildup. If you're not the kind of person that will test water frequently in order to notice when they stop working, however, this is a dangerous choice.

They're also quite bulky (most people only use them in hidden sumps) and reasonably expensive in the long run.

Is Your Filter Noisy?

Discussions of noise when it comes to filters often come down to personal preference.

Some styles and models of filters are certainly particularly noisy, but most times the only people who need be obsessed with noise have very particular requirements for their tanks (something that's in a quiet bedroom room or entertainment room, for example).

From a personal perspective, I know that when I first started to look at new filters years ago I was initially quite obsessed with finding the quietest option I could. I now don't care nearly as much since the gentle hum of a power filter no louder than the fridge or a soft stream of bubbles from a sponge filter doesn't even register (unless it stops...where the LACK of noise immediately alerts me that something is wrong).

The filters that are particularly prone to being noisy are the cheaper models of hang on back, since the filter is a power filter and the motor is externally exposed (outside of the water chamber). Canister filters are also power filters but tend to be hidden away in cabinets, and sumps and internal filters use motors but the noise is muffled by being submerged in the tank.

Especially if you have the misfortune of owning one of the "biowheel" styles of filters, you'll be lucky not to go mildly insane from the terrible grinding and whirring sounds most of them make (apparently this is adjustable, but I don't have that sort of patience).

If you pick up a used filter that seems a little more noisy then it should be or one of your older filters starts to rattle or whir more than it used it, know that the better models of filters will often offer inexpensive replacement parts to rebuild older units. Replacing those parts will have the advantages of both reducing noise AND improving filter performance.

The usual suspects for noise are damaged or dirty impellers (the part that moves the water) or bends or wear in the shaft that the impeller spins on. Sometimes a little sludge removal can clear this up as well, so do give that a try before buying a new part.

If you're considering air operated filters like the sponge filters I strongly recommend, the usual suspects for noise will be the air pumps which drive the sponges. Some air pumps are quieter than others, so check the reviews of ones you're considering to see what real people think of the noise level (they all claim to be quiet).

If your pump isn't overly noisy but the bubble stream is a little distracting (I find them soothing), adding an air stone to the inside of your sponge (if possible) will reduce the size of the bubbles it produces and make a noticeable difference.

Never Clog Plastic Airstone By Aquarium Coop

These plastic "Never Clog" air stones sold by Aquarium CoOp are a nice, easy to clean option for producing a fine bubble stream

Does Your Filter Contain Cartridges That Are Costing You a Lot of Money Each Month?

Aquarium filter cartridges are usually some variation on a plastic tray containing activated carbon and covered in a flossy felt-type material.

"Cartridge-style" filters are filters designed to take those cartridges (logically enough).

Although they are perfectly capable of getting the job done filtering an aquarium, if you follow the directions and replace the cartridges on schedule you're not only throwing money down the drain but also potentially causing your aquarium to re-cycle each time (resulting in ammonia or nitrite spikes).

Although the usual claim for making you replace the cartridge is that they're only looking out for the health of your aquarium, the brutal truth is that the system is actually designed to make money for them at the expensive of your sanity and pocketbook.

By placing the activated carbon pellets inside the filter floss layer, for example, they make sure you can't replace one without the other. If a cartridge was designed to be replaced without touching the floss layer, you'd have a more serviceable system.

 The floss (like the sponge material used in other filters) can essentially be used forever, however, needing only a light rinsing in old tank water during each water change to clear out debris.

The carbon pellets, on the other hand, will exhaust their ability to pull medication and tannins from the water relatively quickly (especially given the small quantity found in a cartridge).

Even all that said, most aquariums don't actually even need carbon. If you're not trying to remove medication, and you don't have yellowish tannins from driftwood in your tank (they're harmless by the way), you don't need carbon.

You actually need to remove the carbon if you ever do want to treat you fish with medication, or you're wasting your money and getting no benefit. If you ever want to pull the carbon to add a medication, you have to somehow dig it all out of the gooey floss mess without having the entire thing disintegrate.

If you give up and throw the whole mess out (or replace a filter cartridge on the schedule they tell you to), you also throw out most of your filter bacteria and cause a cycle spike right when your fish are already sick.


So why would anyone run a cartridge filter then?

Personally, I have no idea. Unless you were looking to throw money down the toilet and cause long-term hassles with medications, there really isn't a good argument I've heard for doing so ("convenience" seems a bit of a stretch compared to "never").

Some filters are big enough to run multiple cartridges in order to let you replace one while the other does most of the work, but that will still tend to cause a mini cycle spike and doesn't get you away from the option of not running carbon if you don't need it (or want it, with medications).

The limitations and running costs of cartridge filters are so great that if you ever come across a website or store employee that actually recommends using them, I'd be strongly suspicious of whether they've ever used them as they claim, or if they actually have your best interests at heart.

So what do you do if you have a cartridge filter?

The easy option is to replace it with something like an AquaClear or Tidal hang-on-back filter (Aquaclear being by far my favorite of the two).

Both of those filters try to get you to replace the sponges and bio-rings periodically as well, but that recommendation can be safely ignored with regular gentle rinsing. The also come with carbon in bags, but the bags can be stored until needed and replaced by extra foam and/or rings.

If you don't have the money or inclination to go out and buy an entirely new filter, one option that's becoming more well known is what's known as "hot rodding" filters, or changing the way they're normally stocked with media.

In a nutshell, just because your filter is designed to used with cartridges, doesn't mean you HAVE to use them

Here's a quick video rundown on the subject:

If you can't get a new filter or just want to improve the efficiency of one you already own, there are ways of doing so inexpensively.

Add An Intake Filter To Your Goldfish Aquarium Filter

The first and easiest step to improving the overall efficiency of a filter is to add what's called an "intake sponge".

Large Intake Sponge

An intake sponge is a hollow tube sponge that's closed on one end, and is sized to be able to slip over the end of the tube that the filter uses to draw water from the aquarium. They can be added to most hang on back, canister, and sump filters (or anything similar that draws water from the aquarium into a separate filter).

Most easily available from Aquarium Coop, installation goes something like this:

Watch Cory explain how an intake filter is supposed to work

An intake sponge has a few purposes:

  1. to increase the available area for filter bacteria to colonize (increasing the capacity of the filter)
  2. to prevent fry, small fish, and fish with long, flowing fins from being sucked against the intake and damaged
  3. to create an easy to clean "prefilter" that dramatically increases the time required between service days for main filter (especially handy for fiddly canister filters)

In case Aquarium Coop is out of stock or you can't directly order from them (US only), feel free to do what I do and make your own intake filters from blocks of AquaClear replacement sponge. My favorite is to stock up on a few extra AQ110 sponges:

...which I just cut to size with a steak knife from the kitchen. If you cut a slit in the end with the knife, the foam should just slip on the end like a commercial intake sponge. The foam is nice and coarse (you don't want it being super fine or it'll prevent water getting to the filter by clogging quickly) and lasts forever.

Hot-Rodding the Media Tray In Your Aquarium's Filter

Since you just picked up the 6 pack of sponges from the link above anyway, you're also perfectly armed to finish off the media bay in your filter (the place that the cartridges formerly went).

With a few minutes work with that same steak knife, you can completely fill the inside of the filter with Aquaclear sponge. You now never have to buy another cartridge again, you've dramatically increased the capacity of the filter you already own, and cleaning it simply entails saving a bit of water from each water change and gently rinsing the foam filter inserts in it.

BioWheel Filters For Goldfish Aquariums

As a final note, I also think that the hassle, cost, noise and mechanical silliness inherent in the "biowheel" filters is basically the same problem as a cartridge but in a slightly different package.

The wheels are expensive, most people find they stop turning within short order, and the mechanism is messy and noisy.

If personally forced to use one of those filters I'd dump the wheel, fill the filter with replacement Aquaclear sponges, and add an intake sponge. It would then have most of the benefits of a better filter like an Aquaclear, none of the silliness of buying replacement cartidges and wheels, and would have the advantage of letting me use the filter I already own..

Does Maintaining Your Filter Take Too Much Time, or Make Too Much Mess?

Every style of filter is a bit of compromise in one form or another.

Some are a little loud but cheap (hang on back filters), some are messy but reliable (sponge filters), and others are expensive and fiddly but discreet (canister filters).

As a result, if you find your filter to be too much of a pain to service, consider looking into alternatives.

Does Your Filter Use Too Much Power?

Although this tends not to be as much of a problem until "multiple tank syndrome" takes over and your house fills with aquariums, it's definitely true that some filters use more electrical power than others to run.

Those small costs multiplied over many aquariums can start to add up, so owners of many tanks can have slightly different concerns than owners of just a single tank or two.

On a related note, if you go right over the deep end and decide to set up a dedicated fish room full of aquariums, two limitations you'll quickly run into are cost and a lack of electrical outlets.

Cost is fairly self explanatory (many inexpensive filters cost less than many expensive ones), but finding room for 30 electrical plugs without blowing the breaker every 5 minutes is a serious concern.

Typically that's why large fish rooms tend to run on air (sponge filters), and my fish room is no exception. One large pump can run potentially hundreds of tanks, and only a single plug is required for all.

Is Your Water Clear Enough For Your Preferences?

Although not everyone needs their aquariums to be absolutely crystal clear (that being especially challenging with goldfish), this is nonetheless an important concern for a lot of people.

Some filters will accommodate adding fine water polishing pads, the addition of cleanable filter socks, or are even available in super high density to pull out finer particles. Using any of these will result in mechanically clearer water (they function by physically filtering out fine debris that makes water cloudy) at the cost of having to be cleaned or replaced significantly more frequently than coarser or less dense versions.

"Chemical" options are also available to improve water quality, and that's where products like activated carbon:

...or SeaChem's Purigen:

... can come in.

These products won't do much for removing large chunks of debris that are better handled by a filter, but they can go some way to removing ultra fine particles.

Where they really shine is removing the yellowish tannins that can come from having driftwood in your goldfish aquarium (just be careful to choose smooth pieces that won't catch long fins).

The tannins themselves are actually quite beneficial for the fish themselves, but most people seem to dislike the look, and the above two chemical media options will remove them on an ongoing basis.

None of the above will do anything to remove ammonia, nitrite or nitrate (water changes still required), but they also won't remove helpful plant fertilizers either.

Are There Any Dead Spots In Your Aquarium Where The Water Isn't Moving?

Dead spots in your aquarium where the water isn't moving properly are usually caused by filters that don't move enough water, or aquariums that are long enough or deep enough to really need a couple of filters to do the job properly.

The reason that you should be aware of flow patterns in your aquarium is because they can end up causing issues with temperature distribution, can prevent fish wastes from being processed by the filter, and can cause debris to accumulate in areas where it can't be swept up by the filter.

There are a few ways you can tell if you have a dead spot where there isn't enough flow:

  1. you can notice where plants or soft decorations aren't moving like they are in other areas of the tank
  2. you can notice if fish tend to always sleep in a single area (tends to be the lowest current)
  3. particularly if you run bare-bottom aquariums, you can notice if mulm and waste tends to pile up in one area or another.

Fixing these dead spots is a reasonably straightforward exercise, thankfully.

Optimizing Flow for Canister Filters or Sumps

If you have a canister filter where you get to choose where the water pickup and filter return are located, then playing with different arrangements can help here. Usually most people just space them as far apart as possible and never think about it again, but that's not alawys the best setup for all aquariums.

Optimizing Flow for Hang On Back and Internal Filters

For hang on back or internal filters, most people don't realize that they're designed to have the water exit the filter at the top, flow across the surface of the tank, then drop down the front glass and return to be picked up by the lower intake pipe.

As a result, if you suddenly jam a bunch of driftwood, three ornaments, and a bubbling treasure chest between the filter and the glass, you can end up with dead spots.

A clever rearrangement of decor can sometimes help here to remove those low flow areas, or the output power of most filers can usually be modified as well.

If your tank is particularly deep and the intake tube from your filter doesn't reach close to the bottom, you can dramatically increase the overall efficiency of the system by buying an intake tube extension (like this).

If you remember the concept of "water out at top, water in at bottom", you'll see why that's beneficial. You can also help the filter suck up debris from the bottom of the tank where it's easier for you to remove it during filter servicing.

Optimizing Flow for Sponge Filters

Usually sponge filters do a reasonable job of creating flow, but that flow can sometime be a "dispersed" in nature.

Lots of people solve the problem by just adding another sponge filter in the low flow area of the tank (say, every two feet on a long tank), but another option is to DIY an elbow onto the lift tube for the sponge filter.

By adding the elbow you can direct the outflow of water into other areas of the tank, rather than just letting it flow upward as it normally does.

The bubbles themselves will still rise to the surface, but the water flow they're pushing along will be slightly more directional then before (and can remove dead spots).

Adding A Small Powerhead or Pump To Remove Dead Spots

One uncommon (but very effective) option to remove dead spots is to add an extra small internal pump to change the flow of water in the tank.

Owners of goldfish that don't have long, flowing fins can use this style of pump and gain the advantage of being able to easy adjust and redirect its flow:

...but owners of those long finned varieties should probably avoid them in case their fish end up with shredded fins.

As an option that's a little safer for those cases, I quite like these little guys from Hydor:

They're not going to break the bank, compact and easy to hide, adjustable, and can easy have a small pad of sponge added over the intake to prevent curious goldfish getting in trouble.

If you want to really optimize your system, consider hooking any auxiliary flow pump to a simple mechanical timer like this:

Using a timer means that you can turn it on an off at different times of the day, and you don't end up just letting debris settle in a new spot like might happen if it was running all day long.

If you have the timer turn off at night, you can even let your fish have a quieter tank to sleep in. You can't do this with filters or you risk destroying the bacteria they contain, but you can certainly do so with pumps with no worry.

Do You Have Trouble Keeping Your Goldfish Tank Cool?

Although there are plenty of websites out there claiming that because goldfish are "cold water fish" you need to keep their tanks at some fairly cool temperature, the truth is that they're actually better described as "cold water tolerant" moreso than "cold water fish".

After all, breeders like us have found that the temperature where goldfish seem to grow the best while still staying healthy is 73F, and that's not exactly what I'd call cold (it's actually on the low range of tropical temperatures for a lot of fish).

That said, when temperatures creep north of 83-86F, a lot of goldfish seem to show signs of stress (it's where they start to have dangerously altered metabolism as well). As a result, if you live in a place where it gets particularly warm, you may end up needing to be careful about heat during those periods.

Although there are lots of ways to combat the heat, one contributor to warm tanks that most people don't consider is the motor in their filter.

The motor in a big Fluval FX6, for example: rated at 43W. Since the water from the aquarium needs to pass through the filter, and the motor attached to said filter kicks out waste heat like any other electrical item, that's almost like having a 43W heater running in your tank 24/7 (the actual value will be a little lower, but the concept is still valid).

In cases where you're having extreme difficultly keeping your aquarium cool, switching to something like a sponge filter (no motor) or a filter with an external motor (hang on back) can potentially make that job easier.

Do You Just Want a Change?

Sometimes you just want to try a different filter, and that's perfectly okay!

Maybe you see people talking about the hot new popular filter on the market and want to give it a go, and maybe you want to try going super old school to see if things are actually better now than they used to be.

Sometimes you get a filter with a kit or used aquarium that seems okay for a while, or maybe you inherit something from another hobbyist that looks really interesting.

Those are all perfectly fine reasons to change, and as long as you handle the transition properly (more on that here), there's no issue doing so.

What Are the Most Commonly Available Styles of Goldfish Aquarium Filters?

Sponge Filters

Sponge filters are air-powered filters that are basically giant weighted blocks of foam with a hollow tube inside.

A stream of air is injected at the bottom of the sponge, and the rising air forces water up the filter tube. The displaced water from the core is replaced by water drawn through the foam block, and bacteria populating the foam filters the water biologically as the sponge itself removes small particle mechanically.

As a nice bonus, the rising stream of bubbles helps to keep dissolved oxygen levels up in the tank as it mixes the water and breaks the surface, and as a result sponge filters are one of the best options for providing this benefit.

Almost contrary to their simple and inexpensive nature, sponge filters are extremely capable filter systems that most large breeders, wholesalers and even some commercial stores rely on.

The combination of low cost and the ability to run dozens if not hundreds of sponges off a single appropriately sized pump makes for a reliable filtering system that's easy to maintain.

Sponge filters are available in various "porosities" of foam from most brands, meaning that there are coarse and fine foam densities available. Coarse filters won't grab super tiny particles and therefore don't require cleaning as often, where fine versions do a great job of polishing water but will clog quite a bit more quickly.

From a hobbyists perspective, those same benefits can also be had at a much smaller scale. Modern air pumps are simple, reliable, and inexpensive, and rarely have issues restarting after power outages.

Should a power outage occur, battery powered air pumps can quickly replace wall-plug versions, and can run for hours on replaceable batteries.

Even in the case where those aren't available, the fact that the sponge is submerged in the main tank means that this style of filter is the least likely to have filter bacteria die off since it's always exposed to the oxygen found in the main tank.

Sponge filters don't cause heat buildup in tanks either having no electrical parts in the water, and generally air pumps consume very little power. Combined with having no moving parts to break down and replace, sponge filters are one of the most cost effective means of filtering a goldfish aquarium.

Sponge filters are also literally the perfect media for raising baby goldfish fry since there are no intakes leading to pump impellers for the newborn fish to struggle against. Flow levels can also be easily turned way down to ensure that those first tentative journeys to the surface to fill their swim bladders happen uneventfully.

The biggest downsides of sponge filters are that they're relatively hard to conceal with their rising string of bubbles and a little bit messy to service (very easy though, mind you):

Servicing a Sponge Filter

In a nutshell, to service a sponge filter all you need to do is to take a large Ziplock freezer bag, fill it with water, and gently slip the sponge fitler into the bag. If you wait to disconnect the air line until the sponge is contained, you'll avoid releasing extra debris back into the tank.

With the filter bagged up, remove the uplift tube, weight and center tube so that only the sponge part itself remains within the bag.

Squeeze the sponge repeatedly until no more debris can be removed, then dispose of the cloudy water in a houseplant or down the drain. Fill the bag and repeat the squeezing process until the water doesn't darken anymore.

Once the sponge is clean, reassemble it (except for the uplift tube, if used), slide the uplift tube up the airline, connect the airline to the sponge, then connect the uplift tube to the sponge.

Replace the filter in the tank and rearrange any decor to hide it (if desired).

A video tutorial on how to service sponge filters by Cory of Aquarium CoOp

Likewise from a noise perspective, some people find the sound of air bubbles soothing while others find it annoying. Poor air pumps can also be a little bit on the noisy side, but thankfully most options don't have this issue. If you're looking for an absolutely silent filtration system (though I'm not sure this exists), air systems might not be your best option.

Using chemical filter media with sponge filters is a bit of a challenge since there are no media bays to add bags of activated carbon or purigen to, but since a sponge filter owner is already armed with a handy air pump, this problem is easily solved by having an inexpensive box filter around (the box filter can share the same air pump in most cases).

In fact, the combination of coarse sponge filters for biological filtration and box filters filled with water-polishing filter floss is how I run all of our display and breeding tanks here at Arctic Lights. The floss simply gets replaced at each water change, and the tanks stay very clean even stocked heavily with growing fish.

In terms of sizing, just from a sanity and ease of replacement perspective all my tanks use the same size filter ("rated" for 60 gallons, though I have no real idea how that's quantified) except for the very tiny 5 gallon fry tanks which use a smaller size to save space.

I personally suspect that most sponge filters are underrated, actually. When you compare the volume of a sponge to the media capacity of most similarly rated power filters, the sponge filter often comes out on top. That said, the best way to tell is to check your ammonia and nitrite levels after the tank is cycled, and if you can't get both of those to 0, you need to clean your filter more, upsize your sponge, or add a second one.

In order to have filters to steal for quarantine tanks and to make sure they're not plugging up weekly (I run heavily stocked breeder tanks), I've settled on the following arrangement:

Volume of Tank Number of "60 Gallon" Sponge Filters Number of Box Filters (Optional)
10-20 gallons 1 1
29-65 gallons 2 1
75-90 gallons 3 1
125 gallons 4 2

There's no real drawback to running multiple sponge filters other than the extra room they take up in the tank, and by having multiple ones going you always have extras available to start up a new aquarium or to setup a quarantine or hospital tank. Keeping those last two running all the time is a bit of a pain, and this is a very easy solution (though I'd advise you to sterilize the filters before adding them back if used for hospital service).

Tips for Using Sponge Filters

In terms of tips for using sponge filters, while they're fundamentally fairly simple things I do have a couple of hints that can make your life even easier. None are required, but all have their uses.

First of all, if you want to make using submerged air-powered filters of any description, do yourself a favour and purchase silicone air tube. It's slightly more expensive than the cheap, clear tubing that most stores sell, but doesn't harden maddeningly like the clear varieties all seem to. When you're elbow deep in a cold aquarium and NOT struggling to reattach a loose airline that won't stretch to fit over the filter barb, believe me, you'll thank me for the silicone tubing.

Next, the thing most people end up being concerned by the most when outfitting an air system is choosing an air pump that's powerful enough to get the job done, but is completely quiet. That's a bit of a balancing act.

Since a pump that's not powerful enough will struggle to push air to the bottom of an aquarium, I recommend you purchase a reliable pump that's slightly more powerful than you need.

You'll not only have enough air to drive the sponge filter you buy, but you'll also have reserve capacity to add a second one should the need arise.

An adjustable air pump is handy as well in case you want to be able to tweak the flow rate through the pump. More airflow will draw more water through the sponge, but will create more bubbles and current in the tank. If you notice the goldfish getting blown around (or are using this in a tank with very young fish), know that a slower flow rate is still more than capable of getting the job of filtering the aquarium done.

Getting a pump to be completely silent is almost impossible, but "noise" from an air pump is also something I think the hobby blows slightly out of proportion. Most pumps are certainly quiet enough not to be disturbing (4 Fluval air pumps are still quieter than my fridge or dishwasher, for example), and only something like a bedroom or music room might require something completely silenced.

Personally, I find that the Fluval Q series are a nice compromise between power, price and volume. We own at least 8, and they've been holding up well.

The Fluval Q1 has two air ports (and thus doesn't need a splitter to run two sponges), and the Q2 is adjustable and more powerful

In the event that they get a little old, you can even purchase a pump rebuilt kit, and with 5 mins of work be back to essentially a brand new pump (the diaphragms on all air pumps of that style will eventually need replacing).

This module (click to view) is a very simple install should your Fluval air pump ever start to lose output pressure.

The only way to avoid needing a rebuild kit eventually (and they're very inexpensive mind you) is to get a linear piston style pump, and those start at a few hundred dollars for most models.

If you need to run a few items at once and your pump only has one outlet, a splitter can come in handy. This set below (click the photo to see details) even comes with a few check valves which is handy (though one on the line from the air pump is more than sufficient).

An adjustable air splitter manifold, suction cup mounts, and check valves to prevent back flow

If you want to minimize the noise from a sponge filter to the greatest degree possible and potentially even increase their efficiency, some sponge filters are capable of housing an air stone inside of them.

The air stone will generate a finer stream of bubbles than the sponge filter alone, and the sound of bubbles gurgling and popping makes up most of the noise that the tank itself generates.

As a further noise reduction option, and one that can help those in hard water area of the world minimize mineral buildup from spray, LR Bretz pioneered the use of drink cup lids slipped onto airline through the straw hole and left floating on the surface of the water.

Although the air line needs to be relatively vertical and tight for this to work, the bubbles from a setup so arranged burst on the underside of the drink lid rather than exploding from the surface of the water. The clear drink lids from cold drinks at Starbucks work wonderfully for the purpose.

If you have small fish that tend to go exploring and might be prone to getting stuck, or just hate the hassle of having to attach them every time the sponge gets hooked up, the little section of uplift tube that comes with most sponge filters can be safely discarded.

Doing so will slightly reduce the water flow rate through the filter, but will increase the "dwell time" (or the time that water is in contact with the media), so the effect on actual filtration end up being minimal.

Conversely, if you want to increase the water flow rate through the sponge you can lengthen the uplift tube using plastic tube purchased with the same dimensions. This can be handy for those who want to polish their water to an extreme, remembering that the sponge filter will end up needing more frequent cleaning that way.

If a 90 degree, right-angle elbow is added to the top of the uplift tube the water flow from the sponge can be directed into dead spots and circulation patterns can be changed to encourage debris to be swept back tot he filter. This is usually a DIY option, but can be very handy particularly for bare bottom tanks.

As a final note, do be sure to add a check valve in the air line used to feed the sponge filter if the air pump is located below the tank. This will prevent water potentially backing up and siphoning out through the pump in the event of a power outage. We keep out air pumps on top of our tanks for this purpose and for ease of adjustment and service, but most people tend to prefer them hidden in cabinets to further reduce the small amount of noise they generate.

Internal Power Filter

Internal power filters contain a block of media and a pump, and generally circulate water from the bottom of the filter, up through the media, and out across the top of the tank.

Generally this style of filter is most commonly associated with small tanks, and more specifically cheap tank kits where all accessories are included with a new tank. There are some quality aftermarket versions available as well, however. and some even offer outflow hoses and directional flow control similar to canister filters.

From an advantage perspective, internal filters are certainly one of the safer choices for those that have delicate hardwood floors and need to minimize the risk of tank flooding. After all, internal power filters are somewhat like a hang on back filter where the media chamber is contained within the tank. As a result of this arrangement, they don't have the risk or flooding that hang on backs or canisters have where a leaking or plugged filter can result in water on the floor.

In a power outage, internal filters that allow some degree of natural water flow in and out of them can allow bacteria in the biomedia to stay a live longer than a canister or hang on back version. This is because the filter media can share the oxygen levels found in the main aquarium water.

On the disadvantage front, the biggest drawback of an internal filter is found by it using up space within the aquarium. Particularly since it's normally used in small aquariums, internal filters can end up using up a fair amount of volume (and are extremely difficult to completely conceal with hardscape materials or plants.

Being inside the aquarium can also make servicing the internal filter messy and disruptive, especially for versions that must be removed to be cleaned or serviced.

Another disadvantage of an internal filter is that the motor is inside the aquarium, so the waste heat it generates transfers entirely to the water. This is generally not a concern until the hot summer months come, and cannot be eliminated or reduced with this style of filter.

Next, although the intakes of some internal filters are reasonably well guarded, they can also be quite challenging to protect fry against. Sometimes a block of foam can be wedged between the intake and the bottom of a tank, but this gets messy if aquarium substrate (rocks or sand) is used.

Lastly, because of the drawbacks mentioned above, relatively few internal filters are available on the market, and they tend to be limited in size and features. That's not because the filters can't get the job done, but because the appeal to most hobbyists tends to be limited compared to other styles.

Optimizing an Internal Power Filter

As an easy filter style to work with, most of the optimization options for this style of filter simply come down to media choice.

Firstly, if your internal filter uses cartridges, they can be safely ditched for a foam or bagged biomedia setup similarly to what is described in the sections above.

If you've already done this, or your filter doesn't use cartridges, media selection is simply a matter of either filling the filter with sponge, or arranging the media so bio-rings or other biomedia are protected by a debris absorbing layer or sponge or filter floss.

If Purigen or carbon are used, this media is added to the top of the filter so that it encounters only the most debris-free part of the water flow and therefore lasts the longest.

From a flow perspective, most of the internal filters operate in a "water in bottom, out top" fashion similar to a hang on back filter. As a result of this, all the flow optimization tricks covered in the hang on back sections are equally applicable here.

Hang On Back (HOB) or Hang on Tank Filter

Hang on back filters are generally one of the first "upgrades" a hobbyist makes to their first aquarium. This is because hang on back filters (on average) tend to give a lot of bang for the buck, and there are a number of solid choices available at a reasonable price that outperform cheap "starter kit" filters.

A hang on back filter is basically a box of media hanging on the back of an aquarium, connected to the tank by an intake pipe and a motor. Once "primed" (filled with water), the intake tube takes water from the bottom of the tank, passes it through a block of filter media or cartridge of some description, and shoots it back into the tank at the surface of the water.

Since the hang on back media compartment sits outside the aquarium, it has the advantage of not using up real estate inside the aquarium. As a result, quite a large filter can potentially be used on a reasonably small tank as long as care is taken not to create too much flow (current) within the tank.

Hang on backs tend to be simple, easy to clean, spacious in capacity, easy to customize the media inside of, and easy to "hot rod" (see this link for how to do that). As a result, they're a very flexible filter type, and as a result are justifiably very popular.

As a downside of having the media chamber behind the tank, however, an aquarium using a hang on back filter can't be placed directly against a wall (space needing to left for the media chamber). If space is not left to access a rear-mounted hang on back they can become messy to service as well, but even a small allowance of space on the side (or a small aquarium) makes this point largely a non-issue.

Mounting a hang on back on the side of the aquarium is certainly possible, and is actually better from a flow perspective, but can make arranging a traditional front-to-back hinged aquarium lid hard to use.

Another downside of a hang on back is that they only contain a limited amount of water inside the filter itself, and therefore in the event of a power outage the life of the bacteria in the media is limited by the amount of oxygen in that small amount of water.

On a related note, this style of filter more than any other seems to have the odd issue with occasionally not wanting to start after being turned off and on again. This is a result of the simple motor designs not wanting to self-start, and is usually easily solved by using a long bamboo skewer or similar. This (and finding enough power outlets) is largely why owners of fish stores or home fish rooms often dismiss hang on backs as a filtration option, since going through tank racks looking for stopped filters after a power spike can be time consuming and somewhat critical.

As a result of this, users of hang on back filters in areas of poor power availability (or ones with power spikes) may want to consider adding an inexpensive UPS (uninterruptible power supply) of the style that a lot of computer users add to their desktop systems.

As a final note on setup, do NOT ignore the little "leveling" tabs or screws that are supplied with good hang on back filters.These are included so that a filter that gets a little dirty and starts to clog will overflow INTO the tank if tipped towards the tank rather than out onto your floor if the media chamber is angled backwards. Ignore this step at your peril (been there, got that t-shirt).

Optimizing a Hang On Back Goldfish Filter

Much like an internal filter, much of optimizing a hang on back is simply ditching any cartridges that they demand (link on why here) and filling the media chamber with long-lasting alternatives.

If bio-rings or similar are used, make sure a layer of sponge or floss protects them (install below the bio in most filters since the water flows in an up and over pattern). If you choose to optionally run Purigen or carbon, that goes uppermost in the filter.

Where the hang on back differs from an internal filter is that it has the option of running an intake sponge. These sponges add extra filter capacity by giving more surface for bacteria to grow on, and extend your time intervals between needing to clean out the filter internals. They also completely eliminate the danger of damaging the pump impeller from sand ingestion, which can otherwise be a problem for owners with fine sand substrates.

To optimize the placement of your hang on back, follow this link to the earlier discussion about flow patterns and how to optimize the decor in your aquarium. Just be aware that if you have a very long and narrow aquarium like a 55 gallon, the "out at surface, in at bottom" circular flow pattern they create might mean that it's more efficient to run two smaller hang on backs rather than one large one. The high flow rates required to get enough flow to the portion of the aquarium not directly in front of the filter if only one is used might be difficult for some slower-swimming fancy goldfish varieties to deal with.

As a final note here, be aware that some newer designs of hang on back filters are trying to get cute about adding complexity to their designs by advertising things like "wet/dry trickle chambers".

"Unnecessarily complex" would be the polite version of my thoughts on this sort of hang-on-back filter.

While I have no doubt that they're effective on some level, the added complexity is not only not worth the hassle, it also takes away from media capacity that could otherwise simply be filled with sponge. I'd need to see actual lab data backing up the some increase in capacity using the trickle arrangement vs the same area of sponge before I'd believe it, personally.

Instead, these filters appear to be more of an attempt to justify another style of expensive cartridge system, and if you've read other parts of this article you already know my thoughts($$$) on that.

Canister Filter / External Canister Filter

For one reason or another, owning and using a canister filter on their goldfish aquarium is held up by a lot of people as being the "best" type of filter that you can use. Whether that's true or not will vary based on what you most value in a filter, and I'm certainly happy to discuss the pluses and minuses that come with using a canister filter.

Canister filters at their heart are just sealed containers filled with media and a flow pump, and are connected to the aquarium via an intake tube and outlet tube (or sometimes these are split). Usually mounted under an aquarium in the stand, they're definitely one of the more concealed and discrete options for filtering a goldfish tank.

On the upside, canister filters tend to be very well made on average. This is usually partly because the tend to be very expensive compared to another style of filter with the same media capacity, but also because they need to stay perfectly sealed or they can leak almost the entire contents of an aquarium onto the floor.

Since their capacity is only limited by the amount of space under an aquarium (generally very large) and the size of the hobbyist's wallet, quite a large canister filter can often be fitted to the average goldfish aquarium assuming that the style of filter chosen can be turned down in flow enough not the blow the fish around the tank. This doesn't usually mean the water needs to be changed any less than with another style of filter (nitrate produced by a canister is the same as nitrate produced by any other filter), but it can extend the length of time between servicing and cleaning checks on the filter.

As a downside of being easily concealed and having a generous media capacity, canister filters tend to be the most neglected on the principle of being "out of sight, and out of mind".

Adding to that potential for neglect, servicing a canister filter is often quite an elaborate affair involving tarps, bins and often some degree of cursing or swearing. The filter needs to be shut down and the lines drained, the water inside the canister usually has to be removed or at least reduced in order to remove the debris released when the media is cleaned, and the filter has to be reassembled perfectly to avoid having any leaks when it's turned back on. This last part is often what gives people fits at first until they figure things out, since some canister filter designs can be quite fiddly and involve a number of small parts and rings.

When the filter is cleaned and reassembled (hopefully using a small amount of lubricant on the seal o-rings lest they dry out and start to leak), the aquarist is faced with the challenge of getting the filter to "prime" again before it's turned on. On some designs this is quite straightforward, and on others this can be agonizingly frustrating. If the filter is not primed successfully it will not flow water, and the motor can be quickly damaged if run without proper water flow.

In the event of a power outage most canister filters will restart if they haven't been opened and "lost prime", but if power has been lost for a number of hours they have been known to be one of the worst offenders for "crashing" a tank due to bacterial die-off from lack of oxygen. Since the filter is not open to the air in any form, the water volume contained within a sealed but powerless canister filter contains all the air it ever will to keep the bacteria inside alive. If left long enough these bacteria will suffocate, and eventually be ejected back into the main tank in a puff of debris when power is resumed.

As a consequence of being both well made and well concealed, however, canisters are one of the best choices for areas where absolute silence in operation is required. This is one of the leading arguments for their use.

Since the water volume within the filter is exposed to the heat that the motor generates when in operation normally, canister filters can contribute at least partially to issues that some people have with keeping a goldfish tank cool enough. Some of the waste heat is released into the air, but given there tends to be minimal airflow under an aquarium stand, most will end up in the aquarium in one form or another. With some of the larger pump motors being up to 50W or so, that can be very similar to running a heater of comparable size at all times in your aquarium.

In terms of returning water to the tank, canister filter hoses tend to offer the most flexibility of just about any filter system when it comes to establishing healthy flow patterns within a goldfish aquarium. As long as care is taken not to create so much flow that the goldfish tire themselves out fighting against it, a careful arrangement of intake and outflow pipes from a canister filter will prevent harmful dead spots forming in the tank.

For particularly sensitive fancy varieties like pearlscales or butterflies, some even choose to use an outflow spraybar to diffuse the returning water flow. This does not create the same amount of circulation within a tank, but also doesn't create the strong currents which these generally poor swimmers cannot deal with.

One of the biggest downsides to a canister filter is also found in the fact that they tend to be one of the most expensive options for filtering an aquarium. Although some people see it as almost a status symbol to have the latest and greatest canister filter (or two) on their tank, rest assured that they're paying for the privilege (which with some people, seems to be almost the point).

Optimizing A Canister Filter

Since most canister filters are relatively generous in the amount of room they contain, and even divide this room into handy tray sections, aquarists are left with a number of options for adding media to their filters.

First off, however, it's important to understand whether the flow through your particular filter is bottom-to-top or top-to-bottom, since canisters are available in both varieties (with no real benefit to either arrangement except ease of cleaning for top-down styles).

After determining the flow direction, fill the trays which the water encounters first with foam or filter floss. Coarse foams will not clog as easily and require cleaning less frequently but don't polish the water as closely, and fine foams and filter floss give crystal clear water but require cleaning or replacement more often.

Assuming you aren't simply filling the filter with foam (a legitimate option, and one I endorse), the next trays should be filled with bio-rings or some other ceramic or similar material. These provide the majority of the bacterial growth area, but will clog quickly without some protection.

Lastly, should Purigen, carbon or some other chemical media be used, this should go on the trays which are closest to the water outlet pipe.

As a last step which is not commonly done but which can extend the time between annoying servicing jobs, I strongly advise using a coarse intake filter sponge over the intake of every canister filter. Not only will it avoid sucking in sand which can damage sensitive pump impellers, it also traps a large amount of debris before it ever enters the filter. Cleaning this intake once each water change can mean some filters may not need servicing more than a few times a year, which can be a real boon to hobbyists that are short on time.

Less Common Goldfish Aquarium Filter Styles

Box Filters

Box filters are extremely simple yet very effective filters which used to be much more common in the hobby, yet now tend to only be used by breeders and hobbyists with an appreciation for simple and low-tech systems.

Working on the same air-displacement principles as sponge filters, most box filters use an air pump to inject a stream of bubbles into a box chamber, and as the air rises out of a separate uplift tube, it pushes water with it. As some water leaves through the uplift tube, other water is drawn in through slits in the outside of the box.

The water drawn in through the slits passes through filter material in the same way it does in other styles of filter, and boxes have the ability to run any media that any other filter can use. This includes chemical media like carbon or Purigen, making them a lovely "sometimes" option to have on hand for those of who run air-powered filtration systems and normally can't run these types of media in our sponge filters.

Box filters are certainly neither sophisticated nor technologically appealing (for those so inclined) but that doesn't mean they do any less of a good job actually filtering aquarium water. In fact, given that they contain no moving parts to wear out or replace other than those in the air pump, box filters are one of the most reliable filters available.

Savvy breeders like Greg Sage of Sage Aquatics even prefer them over other media even including sponge filters, where they use the visible buildup on gunk on the white floss media in clear box filters to help gauge when individual filters need servicing.

Box filters aren't quite as good for small fry tanks as sponge filters are since they can still trap small, newborn fry, but for anything larger than the slits in the filter sides they're very inexpensive, effective items.

For weird shaped tanks, or very low water level tanks, DIY versions of box filters can be an especially nice option.

Optimizing Box Filters

Although most box filters are run on air, there is the option of powering them with pumps of powerheads instead.

This dramatically increases the flow of water through the filter (and therefore filter effectiveness), but also reduces the time between cleanings, brings in the danger that pump intakes create for small fish and fry, and adds another item that can fail.

As a very simple but extremely effective method of filtration, consider doing what I do and couple the use of a coarse foam sponge filter (or more than one, if the tank size is large) with a box filter.

You run the sponge filter for biofiltration (ammonia/nitrite/nitrate), and run the box filter just filled with cheap filter floss and a few rocks to hold it down to polish the water.

Not only do you end up with sparkly clean water with no floating debris, but the system is very inexpensive, flexible, and simple to service (squeeze the sponge filters every few weeks, change the filter floss each week). Highly recommended for heavily stocked tanks where hiding filters isn't the highest concern.

Undergravel Filters

Undergravel filters are an older style of filter that, again, has only lost style points rather than lost any effectiveness. They can be a filter style that particularly excels in returning sparkly clear water in particular, and with the right choice of substrate still remains a solid choice for goldfish tanks today.

An undergravel filter consists of a series of very inexpensive plates that ideally cover the entire bottom of the aquarium. These plates are heavily slotted or perforated with holes, and create a gap about 1/4" high between the raised plate and the bottom of the aquarium. This plate is then covered with an even layer of gravel.

In order to draw water from under the plate, generally two or more riser tubes are located in the back corners of the aquarium. Airlines and airstones are inserted into the tubes and, like the riser tubes on a sponge filter, a rising column of air displaces water and creates from that circulates water from the aquarium, through the gravel, under the plate, up the riser tube, and back again.

As a result of this, the entire gravel surface and plate structure provides a wonderfully oxygenated environment for filter bacteria to grow on and a huge surface area to boot. This style of filter therefore becomes very resistant to disturbances, and allows for very deep water changes without risk of exposing the filter.

That said, undergravel filters (like any) do come with a few downsides.

The biggest challenge with undergravel filters is finding a pebble size that's small enough get proper flow patterns through the filter, but large enough that your goldfish won't try to eat and choke on them. Small gravel sizes that are perfectly safe for small fish can eventually become trouble as the fish grow up, so do try to get a feel for what the mouth of a full grown goldfish of the type you keep looks like before making your final selection.

A lot of the larger gravels can be too  sharp edged for fish that like to dig, and a lot of gravel substitutes like Hydroton will be too light and floaty to stay in place, but with a little looking around a suitable material should be available for most folks.

Likewise, since a lot of goldfish like to dig, making sure that the gravel stays evenly spread across the surface might be a little bit of a challenge. If the plate is exposed in one or two spots, the flow through the entire plate gets disrupted. This is because water will take the path of least resistance, and the efficiency of the filter will drop a bit.

Installing an undergravel filter is very simple, but due to the nature of the filter taking up the entire bottom surface of the tank it's obviously not something that can be added without breaking down and resetting the entire tank. Retrofitting an undergravel filter to an existing tank without that step would be messy and frustrating.

Although a lot of people that have never used undergravel filters fear that they're simply ticking time bomb debris factories waiting to kill their fish, users who have broken down mature undergravel filter setups often find little to no debris under the plate when it's removed (especially if planted). This is because the large, aerated surface works very effectively in breaking down fish waste (which is then removed during water changes).

Some people will claim that the filters are hard to clean, but that can be simply a matter of arranging your setup correctly.

Waste trapped by the filter plate can be seen as a problem, or it can become the perfect fertilizer for rooted plants (delivered right where they need it). Far from being a poor choice like some claim, planted undergravel filters can work very well indeed.

Larger plants like Amazon swords or Madagascar Lace plants may need pre-planned holes cut for their crowns and stems, but normal stem plants and other plants with a finer root structure will find their own way. The gravel and filter plate provide highly oxygenated water right to the roots in either case, and that's gratefully accepted by the plants and results in much better growth than packed sand or regular gravel.

For those that choose not to plant their goldfish tanks, normal maintenance with a gravel vacuum will remove most of the accumulated waste with no issue. Ideally the maintenance is done in small sections at each water change rather than all at once in a single sitting to avoid distrubing the filter bacteria too much, but at worst a few days of cloudy water should be the only result of getting too overenthusiastic with the gravel vacuum.

Although the undergravel filters provide wonderful biological (ammonia/nitrite/nitrate) and mechanical (particle) filtration and can result in the the clearest of water of almost any filter type, like sponge filters they do make it challenging to run any chemical (carbon/Purigen) media. A small box filter is a nice purchase to use temporarily should chemical media be required, and can be run off the same air pump as the undergravel filter if the pump is sized with some excess capacity.

Optimizing Undergravel Filters

For those wanting to really optimize their undergravel filters, a few non-traditional options are available.

Firstly, like with box filters powerheads can be swapped for the air stones if increased flow is required for the filter. This will increase the filtration ability of the filter, but will also require more frequent cleaning as a result.

If powerheads are used, the option to run a reverse undergravel filter setup also becomes available. In this arrangement, rather than drawing water from the main tank through the filter plate, water is pushed down the "uplift" tubes by the pumps, under the plate, and emerges up through the plate into the main tank.

With reverse flow you have a few options as to what to do with the waste. You can either add a prefilter (normally sponge) to the pump or powerhead that will capture the debris before it gets sent down the tube (making cleaning easy):

A Marineland powerhead (left) and intake prefilter (right) allow undergravel filters to be easily used in reverse flow

...or you can direct the waste materials under the plate directly to the roots of any plants growing through the filter plate. Both options place any escaping waste under the filter plate rather than on top of the gravel, and this can make for substantially less waste and reduces the need to gravel vacuum.

Lastly, if you're handy enough in the fishroom to be able to drill your own holes (or have a local company that can help out), non-tempered glass aquariums can be bottom drilled for a waste removal valve. By opening this valve during water changes, the tank is not only easy to drain but the filter plate waste is easily and directly removed without clouding the water. This can be an especially ideal setup if reverse flow is being used, and most of the waste accumulates underneath the filter plate. Uncommon in North America but frequently seen in wholesalers overseas, this option is  highly recommended for making your life as easy as it can be.

 Available Undergravel Filters

Sizing an undergravel filter is simple, since your aim is just to cover as much of the bottom surface of the tank as possible. Small vacant strips are not an issue, and can simply be filled in with extra gravel (aesthetically, leaving that gravel layer at the front of the tank seems to look the best). Measure the bottom dimensions of your aquarium and try to find the best match below (combining smaller plates is perfectly acceptable):

Sumps / Sump Filter

More common in the saltwater world than the freshwater world, and even less-so when used with goldfish, sumps are nevertheless a perfectly appropriate choice to use in your setup. In fact as you start to look at larger aquariums later on (120 gallons and larger), most of the new and used setups of that size will often be drilled for use with sumps.

In a nutshell, a sump is an auxiliary aquarium usually placed underneath the display tank which is used to contain all the filtration and heating equipment for the setup. This has the dual advantage of both adding extra volume to the overall system and hiding all the equipment away from prying eyes (and curious fins and noses).

Although sumps are commonly smaller than the main aquarium, there's no actual limit to the size of the sump other than what can be accommodated by the stand or installation. The only hard limitation of the sump is that it has to contain enough volume to accept the amount of water which will naturally back drain from the display tank when the return pump is shut off. Failure to allow for this volume will result in the sump overflowing.

Since the sump is isolated from the main aquarium, some hobbyists use sumps to grow out small fry or shrimp that would be eaten in the main tank, or to grow plants to help absorb nitrates produced by the main tank goldfish. There's even a line of thinking that has sump lights turn on when the main lights turn off, so plants can continue to operate throughout the day while letting the goldfish in the main tank have a blacked out period of rest.

Sumps have a number of advantages to their use, but also suffer from a few disadvantages. The primary drawbacks to a sump are complexity and cost.

Since relatively few kits are available that function perfectly without the user understanding them quite well, operating a sump requires a level of knowledge on part of the user that some people find intimidating. If you don't enjoy tinkering with plumbing and pumps and have no idea where the piping and plumbing are at your local hardware store, you might appreciate a different style of filter.

Likewise, if you're looking to keep costs down, the various parts that go into creating a sump system will probably frustrate you. I know that the sump systems I've put together have never come in anywhere near budget, but I'll confess that I'm a perfectionist that enjoys tinkering a bit more than maybe I should.

From a running perspective, how the sump and plumbing are setup factor heavily into both how hands-off it is and how quiet it is. As you'll see shortly there are a number of ways to set things up, and each choice can have a big impact on both noise and ease of use. These choices can even factor into how likely the sump is to result in water on the floor; a distinct possibility with a sump that isn't sized or configured correctly.

Lastly, it can be difficult to effectively feed suspended and floating foods to goldfish housed in sumps unless the pump is stopped at each feeding (which is inconvenient). A large cloud of healthy daphnia will be quickly be sucked into an efficient sump, and though most people are unjustifiably terrified of feeding goldfish floating foods, using them becomes challenging if the fish don't eat before the food is sucked into the overflow.

In terms of what goes into a sump system, a circulation pump usually located in the sump takes water from the sump and pumps it into the main display tank. As the display tank fills, water will overflow in a controlled manner into some form of return plumbing (more on this later). The water cascades back into the sump, passes over a heater (if used) and through filtration media in the sump, and gets recycled back by the circulation pump.

The main components of a sump are therefore:

  1. the sump tank itself
  2. the overflow piping which takes water added to the main tank and directs it to the sump as the level rises
  3. the filtration material in the sump
  4. the sump pump
  5. the return line which allows the water to return to the main aquarium

Freshwater Aquarium Sump Styles

Freshwater sumps are usually quite simple compared to the arcane and complicated designs sometimes seen in the saltwater world by nature of the fact that we don't have to accommodate a variety of equipment that isn't used in our setups (protein skimmers etc).

As a result, almost any sump is suitable for use with a goldfish tank.

Common sump sizing says to make sure that the sump is at least 20% of the size of the display tank (ie, a 20 gallon long would be a good sump for a 90 gallon tank), but the bigger the tank that can be accommodated, the better.

This is because bigger sumps tend to be more accessible, and equipment and filter materials are easier to remove and replace.

One thing to consider, however, is headroom under the aquarium stand. A bigger sump may have many advantages, but do leave enough room so you can see into and work within the sump.

Freshwater Aquarium Sump Filtration Material

Media baskets and flow baffles are generally used to make sure water flowing through the sump passes over the filter media rather than diverting around it.

Media baskets are often found under the piping that brings water down from the main tank, and can either be fully submerged or arranged in the air in a "tickle tower" setup (providing more air to the bacteria while still allowing the media in the basket to stay wet).

Flow baffles are usually dividers in the main sump chamber which force water to flow up and down through whatever media is placed within them. These can be foams, filter mats, batting or floss material, chemical media bags, or bio material like rings, balls or even simple kitchen scrubbies (anything bacteria can colonize and grow on).

In a nutshell, almost any media imaginable can be run in a sump, and I've personally even run sponge filters in mine before to polish the water, provide biofiltration, seed extra filters, and oxygenate the main tank discretely.

Sumps can even run water-polishing "filter socks", which are mounted under the overflow piping and filter out debris before it even enters the sump. If the socks get clogged they will simply be bypassed, but the maintenance goal here is to replace (and clean) them before that happens.

If you choose to use the sump as a "refugium" (protected area) to grow out fry or shrimp, make sure to have some protective foam at minimum between the sump chamber and the return pump. This will not only avoid injuring or killing those small animals, but will also provide an additional layer of protection between the pump and any snails that get curious (snail ingestion is a leading cause of sump pump failure).

Freshwater Aquarium Sump Overflow and Overflow Piping Styles

Different styles of overflow plumbing are available, and basically all can be practically used by a goldfish keeper with about equal measures of success (except for the ones I just don't recommend at all).

Vented Drains (Durso Drainpipe, Stockman Standpipe, Hofer Gurgle Buster etc)

If your aquarium is setup for a sump and it only contains a single return line, you're likely to have one of the above mentioned sump return setups.

All piping systems that only contain a single drain line basically function on the same general concepts. These involve taking an oversized pipe, submerging the inlet opening in the main tank, and venting any trapped air through a release. Compared to an open drain (a terribly loud arrangement), this helps to suppress at least some of the noise of gurgling or draining water. 

In order to prevent the surging that happens when a single pipe system starts to siphon (or run without air in free-flow mode), a single pipe setup requires the air vent in order to let air into the system.

This free mixing of air and water oxygenates the water effectively, but can result in a noisier system overall compared to more complex setups. It also provides no leeway for blockage, which can result in floods if the pipe or air vent become obstructed. Lastly, it isn't very efficient from a capacity perspective, which will require larger diameter piping be used to handle the flow from the main tank effectively.

2 Pipe Siphon Systems ("Herbie" Or Similar)

Siphon systems, or ones which move water without ingesting excess air, have the advantage of higher capacity flow (smaller pipes) and much reduced noise.

Due to the increased capacity design they must run at least two unconnected drain lines (the second acting as an emergency line in case of blockage), but they do add a margin of safety should debris, fish, or buildup end up obstructing the main line.

Herbie drains involve a main line with a valve-throttle siphon drain, and a secondary parallel drain line which acts as the emergency mentioned above. By balancing the flow down the drain with the flow incoming from the sump pump, a consistent water level can be set in the overflow section of the main tank.

A two pipe design, consisting of a valve-controlled siphon drain and an emergency line next to it. The siphon pipe has a valve on it — allowing for it to be balanced against the rate of flow from the return pump. This creates a consistent resting water level in the overflow.

If setup correctly the main line will consistently create a siphon each time the pump is resstarted (as would happen during water changes or power otuages), and no manual adjustment should be needed.

The relative simplicity compared to other systems combined with the added efficiency of siphon flow means that a number of commerical tanks come setup with this sytle of piping. If you have see two holes drilled in an aquarium you're considering buying, odds-are it's intended for this style of plumbing.

Bean Overflow / BeanAnimal Overflow

A more conservative but arguably safer and quieter evolution of a Herbie drain is the BeanAnimal style of drain (named after the intenet handle of the inventor, details of which can be found here).

Originally intended for use with a "coast-to-coast" overflow box (a boxed out area with an open top in the main aquarium that stretches across the entire back of the tank), the system has also been adapted to use more compact overflow boxes recently.

This system is a 3 pipe setup and uses the same valve-throttle main drain line, but it uses a secondary line to allow a small amount of water flow and hence act in way that's more resistant to needing fiddling with. Some people do this with a Herbie system as well, but they end up playing with fire in case that secondary line (supposedly the emergency line) ends up accumulating debris.

In a BeanAnimal overflow, the third pipe never allows water flow unless one of the other lines becomes obstructed.

Hang on overflow boxes (Not Recommended)

This style of piping uses an overflow box, but tries to prevent the hobbyist from having to drill holes in the aquarium (required by all the above methods) by using a siphon to equalize water between the display aquarium and an external overflow box. This is the same principle used by "water bridges" which are used to join two aquariums together.

Since a drop in the main aquarium water level and other such events can cause this siphon link to break, this style of plumbing is too risky for me to reommend.

Should this happen and the sumo pump remains running, it will dump the entire contents of the sump over the edge of the main display aquarium and onto the floor.

Some folks try to maintain a siphon through powered means, but this still only provides a dubious measure of security.

PVC pipe only (no overflow box) overflows (Not Recommended)

Many internet forums are filled with PVC spaghetti assemblies that people have dreamed up in order to avoid drilling their tanks.

My recommendation is to avoid them all entirely and either just learn to drill tanks (very simple, I have instructions here) for annealed glass tanks, buy tanks that are already drilled and ready to go, or to hire a shop to do it for you.

DO keep in mind that some tanks (most 55 gallons and the bottoms of larger tanks) will be made of tempered glass, and cannot be drilled. This style of glass is similar to what car windshields are made of and is designed to shatter into small pieces if damaged, but is stronger per inch of glass than standard annealed glass (keeping weight and cost down is why tempered glass is used in some cases).  

Freshwater Aquarium Sump Pumps

Freshwater sump pumps have no special requirements compared to saltwater versions, and are usually an easier-duty application due to the lack of equipment destroying saltwater "crust" which isn't found without high salt levels.

As a result of the huge volumes available for storing media, most people recommend sizing a sump pump to circulate six times the aquarium volume per hour  (as opposed to the 10x rate recommended for smaller filters like hang on backs and canisters). As a result a 100 gallon display tank would need a pump that can deliver about 600 gallons per hour, remembering to look for a pump that can deliver this rate with the lift requirements that are imposed by the aquarium being higher than the sump.

After getting a rough feel for pump sizing, it's recommended to do some research to find a pump that matches the cost you can afford with the noise you're willing to tolerate. Some pumps are powerful but noisy, and others whisper quiet and expensive.

I've personally been using these pumps for years, and continue to use them even to power the water change system in my fish room. They're not whisper quiet, but they're also not particularly loud and are very affordable.

Freshwater Aquarium Sump Return Lines

Doing just what they say, return lines are used by the sump pump to return water into the main display aquarium from the sump.

Placement and design is not quite as critical as that off the drain lines, but care should still be taken to ensure proper circulation within the aquarium and to minimize noise by submerging the line underwater.

That said, the choice to minimize noise by submerging the outfall nozzle does mean that a small amount of water will return to the sump when the sump pump is turned off.

The volume that returns combined with the volume contained in the return line is the reason that a sump should always be designed with extra "head" room compared to what its full capacity would seem to indicate. Failure to leave this room will result in a flood every time power is turned off or lost. 

Optimizing Aquarium Sumps / Overhead Sumps

Much of what's involved in optimizing sumps comes from designing them properly in the first place, and therefore has already been covered in the above sections.

That said, one alternate arrangement that is not commonly seen (but which offers a few advantages) is placing the sump above the display tank in an "overhead sump" configuration.

In this style of setup, the sump becomes much harder to conceal, but it does not need to be sized with the same amount of excess capacity compared to a regular sump. This is because the loss of power does not cause the main aquarium to drain into the sump (the pump being in the main tank rather than the sump).

Servicing tends to become simpler in this arrangement as well, and the main display aquarium does not need to be drilled (this is pretty much the only safe option for placing sumps on tempered tanks).

UV Sterilizers for Goldfish

Although UV sterilizers aren't really filters in the sense that they can't make a dent on ammonia, nitrite or nitrate levels, but they can sort of be considered to be a form of biological filter in that they're wonderful for helping to clear up green water blooms (where suspended algae makes the water completely pea soup green).

Containing a UV light emitting bulb inside the sealed housing and a water pump to circulate water, they function by passing water over that bulb and disrupting the function of organisms suspended in that water (they're safe for fish).

This is the best tool out there for cleaning up green water (which is annoying for keepers but largely safe for goldfish), and can also help prevent disease and help hospitalized fish recover by lowering levels of harmful pathogens in the aquarium water.

UV sterilizers can't do anything for algae growing on surfaces inside the aquarium (they only affect things floating in the water) however, so they won't affect brown diatom algae or anything else on the glass or decorations.

Although some newer filters are coming out with UV sterilizers built-in:

Examples of a Hang on Back filter (by Aquatop) and Canister Filter (by SunSun) that come with UV sterilizers built in as options.


They're also available in stand alone versions as well.

In healthy tanks there's really no reason to run a UV sterilizer at all times, so having the option of removing and storing them when not in use can save replacing expensive UV bulbs unnecessarily.

The built in versions often have a separate switch to turn the UV function on and off, and I'd encourage you to use those only if you see a problem.

My favorite separate versions are made by Green Killing machine, and the two I have on hand have proven useful and reliable. They're a wonderful addition to hospital and quarantine tanks, and are always on hand should green water blooms become a problem.

UV Sterilizers can be incredibly helpful to control green water algae, help sick fish recover, and prevent disease. This style of sterilizer produced by Green Killing Machine is a wonderful tool to have on hand.

Trickle Filters / (Wet/Dry Filters)

Trickle tower filters or wet/dry filters use a steady stream or mist of water taken from the main aquarium to gently bathe the filter media. By having a healthy mix o air and water on the media at all times, this style of filter is said to maximize the biological filtration capacity of the media.

In some extreme cases, these styles of filter are known to even be able to reduce nitrate levels. Since reducing nitrate is the goal of most water changes, this potentially means that the number of water changes (or the percentage of tank changed) could be reduced.

As a result of these benefits, larger versions are very popular in the koi community, and smaller commercial options are available to goldfish hobbyists.

Commercially available trickle towers like this offer goldfish owners the potential of reducing nitrate levels and thus doing fewer water changes


 Since most variations on the filter are simply baskets of media suspended above the tank (as above) or in the outflow from a display tank to a sump, the media that can be used in them is limited only by what you happen to have on hand. All styles of mechanical, biological and chemical filtration media should be possible to use with these filters.

In addition to benefiting the filter bacteria, the highly oxygenated water also has benefits to the goldfish in your main tank. Performing much the same role as an air stone or bubble curtain might, the trickle filter helps to make sure that dissolved oxygen levels are always at their best for your goldfish to benefit from.

While there are undoubtedly benefits to using this style of filter, as always there are a number of drawbacks as well (serious ones, in this case).

The first and most potentially dangerous is that, given a pump is required to keep the media wet and it doesn't sit in a bath of water like with most filter styles, this type of filter is extremely vulnerable to power outages. 

In the event of a power outage, the pump that sprays the media will obviously stop (unless you purchase something like this uninterruptable power supply as a backup).

Power supplies like the above are used to prevent desktop computers from turning off during a power spike or outage. They don't last forever, but they'd run an aquarium filter for a lot longer than a computer!

The media will dry out, and the filter bacteria that lives on it will die. Although this won't happen right away, it's enough of a risk that I know I wouldn't be able to do a long water change or rescape without worrying just a little about the health of my filter.

Next, since this filter works by tricking water in a spray or drip pattern, it maximizes the amount of water surface area exposed to the air and thus encourages evaporation. You can combat the annoying drop in water level in the display tank with something like this automatic top off system:

An automatic top off system notices when the water level in your main aquarium drops and adds water you keep in a separate container.

...but the dramatic increase in room humidity levels may not be something you knew you were signing up for. This might not be a problem if you don't keep a number of aquariums, but monitoring room humidity is something that anyone that keeps tanks should be doing. 

An inexpensive humidty gauge like this can warn you before mold and mildew problems start to affect your home and health.

In order to prevent dust mites, mold, mildew, bacteria issues plaguing your home, ideally you want to keep your humidty levels at 50% of lower. If you happen to live in a colder area, wintertime humidity levels must stay around 30–40% or  lower to prevent condensation on windows and other surfaces.

Fluidized Bed Aquarium Filters / Moving Bed Filters

Generally found more in the monster fish world than in the goldfish world, moving bed filters are containers filled with filter media that is constantly in motion.

By preventing the media from settling and packing down, the entire surface may be eventually coated with filter bacteria, making for a huge available surface and an overall efficient filter design.

Some smaller "internal" versions of the filter are often made from submerged containers like pop bottles or similar packed with media and powered by air (much like a sponge filter), and larger versions based off vats or barrels are often powered by water pumps.

Media can be modern and high tech like the plastic media shown below, or simple and inexpensive like appropriately sized sand:

An explanation of the proper functioning and setup of a moving bed filter

K1 and K2 Kaldness media is commonly used in moving bed or fluidized filters. It's design encourages tumbling in flowing water, evenly exposing bacteria on its surfaces to water and encouraging accumulated debris to shed off.

Deep Sand Bed Aquarium Filters / Deep Bed Filters

Again more commonly found in saltwater tanks, but again equally applicable to the freshwater world, deep bed filters are a very passive style of filter that harnesses the power of nature to do our work.

A quick explanation of deep sand filters as found in the saltwater world. Freshwater systems are equally possible with a bit of work

Deep bed filters use a thick layer of non-compacting substrate to setup an ecosystem which eventually develops the same sort of bacteria that an undergravel filter would.

Stocked appropriately with fish that do not dig (making this a challenging choice for a goldfish tank), these aquarium setups can even eventually develop anaerobic bacteria on their lower levels to convert the nitrates in the aquariums into harmless nitrogen (the largest component of air).

For those looking for a special treat, Ocean Aquarium in San Franscisco operates even their betta sales tanks using deep sand and gravel beds combined with plants.

Boasting that they do no water changes other than replacing evaporation and water lost when fish are sold, their vibrant and lush tanks and healthy fish definitely make a case for deep beds and the power of patience.

A visit to Ocean Aquarium should be on the bucket list of any serious hobbyist!

Algae Scrubbers

Algae is a pest, right? Not if you use an algae scrubber it isn't!

Another filter design that harnesses the power of nature, algae scrubbers live in your tank and provide a perfect combination of light, material and oxygen that encourages algae to grow.

Far from being the controlled insanity that this might seem like at first, by encouraging algae to grow in their little sealed containers, this actually discourages algae growing anywhere else in the tank.

Since the algae grows by consuming the waste products produced by out goldfish (ammonia, nitrite and nitrate), an appropriately sized scrubber also potentially dramatically reduces the amount of water changes that are required (about it can't remove are growth hormones).


Needing only periodic cleaning to remove built-up algae (making room for new nitrate-destroying growth), it's actually surprising that these aren't used more frequently in the hobby.

Algae scrubbers like this one can make a huge difference in the nitrate levels found in aquariums
Typically algae scrubbers are used in conjunction with other filters which provide basic filtration when algae levels are low, but the truly dedicated could probably arrange for all filtration to be done with properly "seasoned" algae scrubbers.

Diatom / Diatomic Filters

More a polishing or mechanical filter than a biological filter, this seldom-seen (in freshwater) filter uses extremely fine filter material to remove tiny particles from water which can cloud a hobbyists view.

In most filters of this style the media used is diatomaceous earth (hence the name), but in all the goal is to polish water to the highest degree possible.

These filters can clean out the water by removing the small dust particles from the water. These are appropriate for the tank where fine size particles exist that causes the formation of the diatomic algae.

Diatomaceous earth is fine enough when used in a filter to remove even the tiniest of water imperfections, resulting in crystal clear water.

How Do I Choose What Filter Style To Buy?

The most important part of choosing a filter is to make sure it's a style that you'll want to live with (and clean) on a regular basis.

That might mean that the filter that's currently popular on the forums or in Facebook groups, or the one that's on sale in your local store, isn't the best choice for your particular situation.

I've given my general recommendations at the top of this article already (link here), but for more specialized requirements here are a few questions to consider.

Is cost a primary consideration?

Good choices:

  • sponge filter
  • box filter
  • hang on back filter
  • undergravel filter

Poor choices:

  • canister filter
  • sump

Is your tank heavily stocked? (more than 1 fish per 5 gallons)

Good choices:

  • sponge filter (easy to clean)
  • hang on back filter (easy to "hot rod")
  • sump (huge capacity)
  • trickle filter (nitrate reduction possibility)

Poor choices:

  • canister filter (hard to service)

Are you willing to do extra work (or spend extra money) for completely clear water?

Good choices:

  • box filter with filter floss + sponge filter combo (cheap and easy to clean)
  • canister filter (large media capacity)
  • sump (can run filter socks and large media capacity)
  • undergravel filter (best at polishing water)

Poor choices:

  • hang on back filter (small media capacity)
  • internal filter (small media capacity)

Does the filter have to be completely silent? (bedroom or entertainment room)

Good choices:

  • canister filter
  • sump

Poor choices:

  • sponge filter
  • box filter
  • trickle filter

Do you have a lot of tanks or a fishroom?

Good choices:

  • sponge filter (cheap and easy to clean, can run on central air)
  • box filter (cheap, easy to clean and shows when needs cleaning, can run on central air)
  • undergravel filter (cheap and easy to clean, can run on central air)

Poor choices:

  • canister filter (expensive, needs power plug)
  • hang on back filter (needs power plug, doesn't always restart without help)
  • internal filter (pain to clean)

Do you live in an area where power outages are common?

Good choices:

  • sponge filter (media has oxygen in power outage from tank, can run on battery air pump)
  • box filter (media has oxygen in power outage from tank, can run on battery air pump)
  • undergravel filter (media has oxygen in power outage from tank, can run on battery air pump)

Poor choices:

  • trickle filter (media dries out during outage)
  • canister filter (expensive, low oxygen for media during outage)
  • hang on back filter (low oxygen for media during outage)

Do you want to breed fish, or have young fry?

Good choices:

  • sponge filter (easiest on fry)
  • hang on back without skimmer (can be made fry proof with intake sponge filter)

Poor choices:

  • hang on back with skimmer (impossible to fry proof)
  • sump (very hard to keep fry out of sump)

Do you want the latest technology and hate justifying your choices to people in forums on the internet?

Good choices:

  • canister filter (currently the "cool" choice for filtration)
  • sump (shows you're "serious")

Poor choices:

  • undergravel filter (will be justifying use of this to people daily)
  • sponge filter (people don't think they're good enough for goldfish tanks)
  • box filter (people don't know what these are)

Do you keep varieties that struggle with a lot of current or have special needs? (bubble eyes, butterfly telescopes, pearlscales)

Good choices:

  • sponge filter (no sharp edges, gentle current)
  • canister filter with spraybar (can diffuse output)
  • sump with spraybar (no sharp intake, can diffuse output)

Poor choices:

  • hang on back filter (without intake sponge)
  • box or internal filter (sharp edges)
  • canister filter with strong output

How Do I Size Goldfish Aquarium Filters? Should I Try To Buy One That Turns Over 10x The Tank Volume Hourly?

There's a common rule of thumb in the aquarium world that the filter system should be able to turn over (pass through the filter) the volume of the aquarium 10 times each hour.

As a result, if you have a filter is able to flow 200 gallons per hour, that should be used on no more than a 20 gallon aquarium (200/10 = 20).

While a nice rule of thumb, and lacking any other information better than nothing, that becomes a little more challenging with our goldfish friends. After all, a good number of them aren't exactly be best of swimmers anymore (I'm looking at you, butterfly telescopes!)

Sizing a filter is actually a balance between making sure all the water in a tank passes through the filter at some point (called turnover rate), and making sure that it stays in contact with the media long enough to be filtered by the filter bacteria there (dwell time).

As a result, you don't really want too high of a flow rate blasting your goldfish around the tank and not getting filtered properly, but also not too low of a flow rate so that waste can build up over time without getting processed.

The deciding factor in this balance is usually the amount of surface area (or amount of media) that a filter is able to contain. A small hang on back filter may need a 10x turnover rate because it can't contain a ton of media, where a huge sump may only need a 6x turnover rate due to the huge amount of filter mats etc that it can contain.

To avoid ending up with a filter that's undersized (or that blows your fish around), try to buy one that either has an adjustable flow rate (AquaClear Hang on Back filters or sponge filters with adjustable air pumps), or can mount a spray bar (just about any good canister filter, especially the Fluval FX series).

An adjustable filter sized at 10x rate but which can be turned down or diffused will provide the most flexibility for your setup.

How Do I Switch My Existing Aquarium Over to Using A New Filter?

If you find that your existing filter is undersized, or you just want to switch over to a new filter, don't just unplug and remove the old one after setting up the new one.

Doing this will mean that your aquarium starts to "cycle" all over again, and you'll be stuck measuring ammonia and nitrite and changing water more than normal.

Since most of the filter bacteria in an aquarium is in the filter media, continuing to use that will prevent problems regardless of what box it happens to be sitting in. As a result, if the filter media (sponge, cartridge etc) from the old filter will fit in the new one, just transfer it over and fill the remaining space with new media.

After a few months the bacteria from the old media will colonize the new media, and the old media can be removed.

If you're changing from one filter to another where reusing the media isn't possible, then running both of them at the same time is almost the only practical choice. Be very careful not to create so much flow that the fish struggle, but this is again where adjustable filters can come in handy. Again, after a few months the old filter can likely be removed (though test the water to be sure before you clean up and sell/store the old "seeded" filter).

You may have heard that "squeezing" the media from one filter into the tank of another can help to cycle an aquarium. This is sort of true in that some bacteria will be transferred, but it's unlikely to be enough to support something like a goldfish. This is better than nothing, but you'll still likely have to end up monitoring the aquarium as it cycles completely.

Getting a "clipping" of media from a cycled filter is a little better in that you're getting a small section of where the bacteria live, but again it's not likely enough to support entire fish populations unless the donated section was very large. You'll cycle for a shorter time, but you'll still have to be careful.

How Do I Handle Filter Changes While Upgrading My Goldfish To a Larger Aquarium?

If you've decided to upgrade the size of your aquarium at the same time you're updating the filter, or you're hoping to reuse the existing filter, there are a few important things to keep in mind.

First off - if you're not changing the number of fish at the same time, you don't have to cycle the aquarium from scratch! This is because one of the golden rules of filters is that the amount of filter bacteria in the tank is related to the bioload in the tank (number of fish), NOT the size of the filter.

To put that in perspective, a little pearlscale goldfish in an 800 gallon paradise needs the same amount of filter bacteria as that same fish in a 20 gallon. The media in the filter from the 800 gallon could be transferred to the filter in the 20 or vice versa, and likely all would be well.

The only difference between the 800 gallon tank and the 20 gallon tank is the challenge of getting all the water in those tanks through the filter, and making sure there are no dead spots. The pump parts of those filters will therefore be very different, but the amount of filter bacteria does not. That bacteria is actually spread over all the surfaces in the aquarium (including all rocks, walls and substrate) rather than just contained in the filter (so our example is a bit of an oversimplification), but I think the point still stands.

As a result, if you decide that the inhabitants of your 20 gallon goldfish tank would really be better off in a 55 gallon, you don't necessarily need to worry about starting the tank from scratch. If your filter can handle circulating the water in the 55 (because maybe it was oversized for the 20), then the new tank may be good to go by simply adding dechlorinated water, moving the filter over, and then adding the fish.

Since bacteria is found on all surfaces of the tank, a brand new tank with the new filter won't be quite as good as a seasoned tank. This is because the gravel, aquarium walls and all other surfaces will not have had a chance to "seed" with bacteria yet. Since MOST of the useful bacteria is in the filter though, it won't be long before the new tank is just as good as the old.

If the filter motor itself is a little on the small side, you can either use the existing filter and add a secondary powerhead or pump to circulate water: 

My favourite adjustable mini pump from Hydor used to increase circulation in an aquarium

...or do what most do and buy a slightly larger filter. As above, if the media from the old one will fit in the new one then you can switch right over to the new filter, otherwise you'll need to run both at the same time for a few months.

In any case, there is very little value in keeping the water from the old aquarium for use in the new one. Very little if any bacteria is found in the water, and most of what you'd be bringing over is all the waste products found in the old tank.

If you've been keeping up on maintenance and your water doesn't contain a ton of nitrate (<30-40 ppm) you can simply make sure the temperatures of the aquariums match and that you've added dechlorinator, then transfer your fish when ready. If you're on a well, you may want to perform the added step of aerating the water a bit beforehand, but otherwise all should be well.

If you haven't been doing your basic maintenance or don't have a test kit, doing a couple of water changes on the main tank before transferring the fish will make sure it doesn't get shocked by moving from dirty to clean water.

In either case, reusing the old tank water is not required.

How Do I Service My Goldfish Aquarium's Filter?

Most of the time, servicing a filter is simply a matter of a making sure the filter is clear and able to do its job. Normal tasks include:

  • cleaning the debris that accumulates in mechanical media (foam etc) by swirling it lightly in a bucket of used aquarium water
  • doing the same for biological media like biorings only if it has started to accumulate debris
  • replacing chemical media that has started to exhaust (Purigen that's changed color), or at whatever interval is recommended by the manufacturer (carbon doesn't show a color change, for example)
  • removing buildup around outlets, impellers, and filter chambers by a light rinse in tank water
  • lubricating o-rings (canister filters) or pump bearings (if directed to)

...and whatever other actions may be applicable to special filters (removing extra algae from algae scrubbers, for example).

In any case, resist the temptation to use water straight from the tap (which may contain chlorine or chloramine), and especially avoid using harsh cleaners like bleach or vinegar. Also try not to let filter parts and surfaces dry out if they're normally submerged, and keep them in a tray of water if required.

Always remember that the filter itself has no magical powers to do anything but remove debris, and that it's actually the bacteria that you careful culture within the filter that actually converts harmful ammonia to nitrite and then nitrate. If you do anything to damage that colony, you may end up having to grow a new one from scratch.

Keep things clear and flowing, but avoid getting too carried away with aggressive cleaning.

How Often Should I Clean My Aquarium Filter?

For the most part, doing a small rinse of the filter media in a bucket of used tank water at each water change will prevent things from getting too far out of line. If the media starts to clog up with debris it can affect the bacteria contained within the filter, so ideally you don't want to wait too long between cleanings.

For mechanical or biological media, unless the filter media is actually physically damaged and the water is bypassing it as a result, or the filter material is breaking down and causing a mess in the tank, there's no reason to replace it (even if the instructions tell you to).

For chemical media replace it as it exhausts (often a color change will be noticeable), or per the instructions included with the product.

Final Thoughts On Goldfish Filters

 At the end of the day, remember that a filter is nothing more than the combination of a way of circulating water in the aquarium combined with a large amount of surface area perfectly set up to grow filter bacteria.

Sales pitches may promise amazing results or offer special features like UV sterilizers or extra oxygenation from bubblers, but at the end of the day they all fundamentally do the same thing.

Your goal with a filter is to find something that can do the following:

  • circulate water within the aquarium
  • filter out debris floating in the water
  • provide media to grow bacteria
  • be part of a system that you personally find easy to maintain and live with

Try to find something that you enjoy working with more than something that works for a friend or random person on the internet, and one that gives you the results that you and your fish need.

If you need recommendations for your particular setup, check back at the beginning of this article for my recommendations.

If you have any specific questions, feel free to drop me a line.

In both cases, I hope you have healthy fish and enjoy keeping them!

Adam Till
Adam Till


Also in The Goldfish Owners Guide

Goldfish Names - Your First Difficult (But Fun!) Decision

by Adam Till

Do you name your fish? If so, do you need some suggestions?
View full article →

Rehoming Fish: What To Do With Unwanted Goldfish

by Adam Till

When know you can't keep a fish but don't know what to do with it, what are your options?
View full article →
Back To The Top