Goldfish filtration: types, sizing and flow

Ask ten goldfish keepers which filter to buy and you will get ten answers, most of them right. Goldfish are forgiving about the type of filter and demanding about two things it must do: hold enough bacteria to process a heavy load of waste, and move a lot of water without blowing the fish across the tank. Get those two right and the box it comes in matters much less.

The three jobs a filter does

Mechanical filtration traps solid waste, uneaten food and plant debris in sponge or floss so it can be removed before it rots. Biological filtration is the important one: bacteria living on the media convert ammonia from the fish into nitrite and then nitrate, which is far less toxic and leaves with water changes. Chemical filtration, usually activated carbon or a resin, adsorbs dissolved substances such as medications, tannins and odours.

For goldfish the first two carry the weight. Goldfish produce a great deal of solid waste and a great deal of ammonia, so the mechanical stage needs to be coarse, roomy and easy to rinse, and the biological stage needs volume. Carbon is optional: it exhausts within weeks, does nothing about nitrate, and is mainly useful for clearing a medication or the tea colour from new driftwood. The cartridges that combine floss and carbon in one disposable unit are a consumable trap; replacing them throws away the bacteria you spent weeks growing.

Filter types and which suit goldfish

The tank sizes given for each type are keepers’ rules of thumb.

Sponge filters are a block of foam on a tube, driven by an air pump. They are cheap, silent apart from the pump, nearly impossible to break, and gentle enough for fry, bubble eyes and recovering fish. Their weakness is mechanical: a sponge traps fine debris but does little about the heavier waste that settles on the floor. On a tank up to about 100 litres a large sponge filter can be the only filter; on bigger tanks it makes a superb second filter and a backup.

Hang-on-back filters clip to the rim, draw water up a tube and return it in a waterfall that agitates the surface. They are easy to service, take no space inside the tank, and most have adjustable flow. Replace the cartridge with a block of sponge and a bag of ceramic media and they become a good goldfish filter for tanks up to around 200 litres. A strong waterfall can still push fancies about; the flow section below covers a baffle.

Internal power filters sit inside the tank, a pump on top of a sponge. Compact and cheap, they suit small tanks or work as a booster, but they take up swimming space, hold little media and often deliver a hard jet.

Canister filters are external units connected by two hoses. They hold more media than most other types at their price, run quietly, and are the usual choice for tanks over 150 litres. The costs are a higher purchase price, a cleaning session every month or two that involves disconnecting hoses, and an outflow strong enough that fancies need it spread with a spray bar.

Sumps are a second tank below the display, fed by an overflow, with the media and equipment inside. They add water volume, hide everything and offer unlimited media space. They suit large tanks, indoor ponds and keepers happy with a little plumbing; for most homes a canister does the job with less risk of a flood. Sumps and large systems covers the plumbing, the drain-down calculation that decides whether one floods, and the cases where it is genuinely the right answer.

Under-gravel filters, which pull water down through a gravel bed, are a poor fit for goldfish because gravel is a poor fit for goldfish. Whatever the type, the filter must be cycled before the fish rely on it; cycling a goldfish aquarium explains the process.

Sizing: turnover per hour

The useful number on a filter box is not the tank size it claims to suit but the flow rate in litres per hour. Many keepers work to a turnover of about five to ten times the tank volume per hour: a 100-litre tank wants 500 to 1,000 litres per hour, a 250-litre tank 1,250 to 2,500. That is a keepers’ rule of thumb. Rated flows are measured with clean media and no hoses, so real output is lower, which is a reason to buy toward the top of the range. The stocking checker will tell you whether a given filter is adequate for the fish you have in mind.

How water moves through a filtered tank

1. 1
2. 2
3. 3
4. 4

1. Intake with a sponge pre-filter. Catches debris before the media and protects fins, eyes and bubble sacs.
2. The filter itself. Water passes through mechanical media first, then biological media where bacteria live.
3. Gentle return. A spray bar or baffled outflow spreads flow so fancies are not pushed around the tank.
4. Whole-tank circulation. Intake at one end and return at the other means fewer dead spots where waste settles.

Turnover Filter flow ÷ tank volume. A 200 L tank with a filter moving 1,200 L/h turns over six times an hour. Keepers commonly aim for roughly five to ten turnovers an hour.

Rated flow is optimistic Manufacturers quote flow with clean, empty filters. Real flow drops as media fills, so size up rather than exactly.

The other sizing number: how much media it holds

Turnover tells you how often the water passes the bacteria. It says nothing about how many bacteria there are, and for goldfish that second number is usually the one that binds. A heavily fed, heavily stocked tank does not fail because the water went round too slowly; it fails because there was not enough surface for a colony big enough to keep up with it.

Manufacturers lead with flow, because flow is the impressive figure and is cheap to increase with a bigger impeller. Media volume is expensive, takes up space, and is often not printed on the box at all. The result is that two filters rated for the same tank, at the same litres per hour, can hold very different amounts of media, and the difference does not appear in any number you are shown.

How to judge it without a figure. Canister filters usually publish basket or canister volume in litres, and that is the fair comparison: between two canisters at the same flow, the one with the larger baskets is the better goldfish filter. Hang-on-backs rarely publish anything, so judge the physical size of the media chamber, and be suspicious of any model whose chamber is shaped around a proprietary cartridge. A cartridge is mostly floss and carbon with very little biological surface, and replacing it monthly throws away most of the colony that lives in it — which is why the section below suggests swapping one out for sponge and ceramic instead.

A keepers’ rule of thumb is to read the tank rating as about half. Ratings assume a lightly stocked community tank, not goldfish, so a filter rated to 200 litres is a sensible choice for a 100-litre goldfish tank. It is the same reasoning the buying guide uses when it compares models on media capacity rather than on flow alone.

Strong filtration, gentle current

Turnover and current are not the same thing. A filter can move 2,000 litres an hour through its media while the fish feel only a drift, if the outflow is spread. Fancy goldfish are weak swimmers; an oranda or ranchu pushed around by a jet will tire, hide in the calmest corner and stop eating well. Single-tailed goldfish are strong swimmers and enjoy some current, but even they need a quiet zone.

The tools are simple. A spray bar, a perforated tube along the back glass, turns one jet into a sheet of gentle flow. Pointing an outlet at a side wall or at the surface breaks its force. A sponge slipped over an outlet, or a baffle cut from a plastic bottle under a hang-on-back’s waterfall, does the same. Keep some of that flow at the surface, because that is where oxygen enters; the aeration guide explains why. Then watch the fish: if they hold position easily and use the whole tank, the flow is right.

What the filter does not do

Worth a section of its own, because it is a common misunderstanding about filters and the reason a lot of well-filtered tanks still read high for nitrate.

A filter processes only what reaches it. The current carries fine particles and dissolved ammonia to the intake, and the biological stage handles those very well. Heavy goldfish waste does not travel: it settles on the floor, behind ornaments, under the filter itself and in whichever corner the flow does not reach. There it decomposes exactly as it would in an unfiltered tank, feeding ammonia and nitrate into the water, invisible to the filter and to you.

Which is why siphoning the substrate is not optional, and why cleaning a goldfish tank is a separate job from maintaining the filter. It is also an argument for a bare bottom or a thin layer of large gravel over a deep sand bed — the substrate guide sets out the trade-off.

Nor does it remove nitrate. The biological stage converts ammonia to nitrite and nitrite to nitrate, and there it largely stops. Nitrate leaves mainly through water changes and, in smaller quantities, plant growth and denitrification. A larger filter therefore buys safety margin and faster processing; it does not buy a longer gap between water changes. The water change guide covers what does set that interval.

And it does not aerate as well as you think. A filter’s return breaks the surface and that helps, but on a warm night in a well-stocked tank it is often not enough on its own — aeration is a separate consideration, and gasping at the surface is what it looks like when the margin runs out.

Media and how to clean it

Fill the filter, in order of flow, with coarse sponge, finer sponge or floss, and then biological media; filter media covers each kind, what to do about carbon, and why disposable cartridges cost you the cycle. Rinse the mechanical stages when the flow drops, which on a goldfish tank is every two to six weeks: squeeze them in a bucket of water removed during a water change, until they are cleaner rather than clean. Tap water carries chlorine or chloramine, which can damage the bacterial film; nitrifiers resist them better than many bacteria, but there is nothing to gain by risking it. Biological media is rinsed only gently, only occasionally, and never all of it at once. Floss is replaced when it falls apart; sponge lasts years.

Pre-filter sponges

A coarse sponge over the intake strainer does two things. It catches large debris before it clogs the filter, which stretches the time between cleanings, and it stops a fish from being drawn against the intake. That matters for bubble eyes, whose fluid-filled sacs can be torn by a bare strainer, for fancies with long trailing fins, and for fry. Rinse it weekly; it will need it. It also reduces flow slightly, which is one more reason to size the filter generously.

Running two filters

A strong filtration upgrade for most goldfish tanks, and often cheaper than the equivalent single unit.

• Redundancy. Filters fail — an impeller seizes, a hose slips, a unit is simply old. With two running, a failure is an inconvenience rather than an emergency, because the colony survives in the other one.
• Staggered cleaning. Clean one filter this month and the other next month, and the tank never loses more than half its bacteria at once. This alone removes a common cause of a mid-life ammonia spike in an established tank.
• Spread flow. Two moderate outflows at opposite ends move more water with less current than one strong one, and they reach the dead corners where waste would otherwise settle.
• A seeded sponge, always ready. This is the reason worth the most. A small sponge filter running in the main tank is a fully cycled filter sitting in storage: drop it into a bucket or a spare tank and you have an instant hospital tank or quarantine tank. Everyone needs one of those eventually, always at short notice, and cycling one from scratch takes the weeks you do not have.

The usual pairing is a canister or hang-on-back for the main work and an air-driven sponge filter as the second. They fail in different ways, which is the point.

Oversizing versus overpowering

Bigger is usually better in the filter itself: more media, more bacteria, more margin when you overfeed or a fish grows. The risk is in the outflow, and the outflow is fixable with the tricks above. So the sensible approach is to buy a filter one size larger than the box suggests and soften its current, rather than buy a small one and hope. Our buying guide to filters for goldfish picks specific models with that in mind, and do goldfish need a filter? makes the case for having one at all.

How to tell the filter is working

A running filter is not necessarily a filtering one, and the difference is not visible.

The chemical test is the real one. On an established tank: ammonia zero, nitrite zero, nitrate present and climbing between water changes. That pattern means the biological stage is doing its job. The testing guide covers how to read it.

The counter-intuitive one is nitrate at zero on a tank that has been running for months. It sounds like the cleanest possible result. With ammonia or nitrite above zero it means the filter is not converting anything — the media has been replaced, the filter has been scrubbed or soaked in tap water, or it has not actually been cycled. With all three at zero in a stocked tank, which is unusual, re-test and look for plant or algal uptake, denitrification or very frequent big water changes. Nitrate is the waste product of a working filter, so its absence is worth a second look rather than a triumph.

Three mechanical checks, worth two minutes a month:

• Has the flow dropped? It falls gradually as media clogs, so compare it against how it looked when clean rather than against last week.
• Is the intake clear? A pre-filter sponge that has not been rinsed will choke the whole filter.
• Does the outflow smell? Faintly earthy is normal. A rotten-egg smell suggests part of the media has gone oxygen-free and needs rinsing now.

When the filter stops

More common than people expect — a power cut, a tripped socket, a seized impeller, a holiday — and the recovery is where mistakes tend to happen.

Flow is what supplies the media with oxygen. Nitrifiers tolerate oxygen-free spells better than most bacteria, and no one has measured how fast a stopped filter’s bacteria die. The risk is the water trapped in a sealed canister, which can turn oxygen-free and foul within hours.

So the response depends on the type of filter and how long it was off:

Filter and time without flow	What to do
Up to a few hours, any filter; or a sponge or internal filter at any length	Restart it, then test ammonia and nitrite daily for a week.
Canister or hang-on-back, more than a few hours	Do not simply switch it on. Open it, rinse the media in tank water, then restart and treat the tank as partly cycled. Test ammonia and nitrite daily for a week.

The reason for that last row is the water that sat in the sealed canister: it goes straight into the tank when the filter restarts.

Noise, and why it matters more than it seems

A noisy filter gets switched off at night, and while it is off the flow and surface movement stop, ammonia builds and the media can run short of oxygen. So the noise is worth fixing rather than tolerating.

• A rattle or grinding is often the impeller: either debris caught in the housing, or a worn shaft. Both are a five-minute strip-down, and a replacement impeller is one of the cheapest parts on the tank.
• Gurgling in a canister is trapped air. Tilt the unit gently while it runs to work the bubble up to the outlet, and check the hose seals, because air getting in means water could get out.
• A loud waterfall on a hang-on-back is the drop height. Raise the water level so the fall is shorter, and the splash mostly stops.
• Hum through the cabinet is vibration transmitting into the woodwork. A folded towel or a foam offcut under the unit removes it.

None of these is a reason to run a goldfish tank unfiltered overnight.