The best filters for goldfish

Many goldfish problems trace back to water quality, and water quality in a goldfish tank is mostly a question of filtration. A filter that would happily run a tank of tetras is often overwhelmed by a single oranda. These are the filters we would buy for goldfish, grouped by the job each does best.

What to look for

Start with the biology. A goldfish converts a lot of food into a lot of ammonia, and the bacteria that make it harmless live almost entirely in the filter, so the number that matters most is how much biological media a filter holds, not how many litres per hour its pump moves. Our filtration guide covers the mechanism.

Turnover still matters. A keepers’ rule of thumb is 5–10 times the tank volume per hour, and because box figures are measured with an empty filter, real flow through media can be well below them, which is why we suggest a filter rated for a tank larger than yours. The stocking checker will tell you whether your combination is reasonable.

Then think about the fish. Many fancy goldfish are weak swimmers by keepers’ accounts, and a ranchu pinned in a corner by a canister outlet is a stressed fish that eats poorly, so look for a flow lever, a spray bar or an air-driven design. Be realistic about maintenance too: goldfish filters clog fast, and a filter you can rinse in a bucket in five minutes will be cleaned. And no filter works until it has been cycled, whatever the packaging implies.

What to buy for your tank

The principles above, translated into a purchase. Sizes assume real water volume rather than the number on the box — a tank holds roughly ten per cent less than its nominal figure once substrate, decor and the gap under the rim are accounted for, which choosing a goldfish tank covers.

Tank and stock	What we would buy	What also works
75–110 L, one fancy	A large sponge filter on a decent air pump	A hang-on-back rated for 150 L or more
110–200 L, two fancies	A hang-on-back rated for 300 L or more	A smaller hang-on-back plus a sponge filter
200–300 L, three or four fancies	A canister rated for 400 L or more, with the spray bar fitted; check its circulation figure too, about 1,000 to 1,500 L/h or more on the five-to-ten-times rule	Two large hang-on-backs at opposite ends
300 L and up, or any single-tails	A self-priming canister rated well above the volume	A canister plus a sponge filter for redundancy
Quarantine or hospital tank	A sponge filter, every time	A second sponge kept running in the main tank until needed
Pond	A pressurised pond filter with UV, rated above the real volume	A gravity-fed box filter with a larger pump

The jump from hang-on-back to canister happens around 250 to 300 litres, and not because hang-on-backs stop working at that point. They keep working; you simply need two or three of them, and at that stage one canister is quieter, tidier and holds more media than three boxes hanging off the rim.

Below about 100 litres the sponge filter is the answer, because the constraint at that size is not the filter. It is the volume, and goldfish tank size explains why buying a better filter for a tank that is too small does not solve the problem the fish has.

Reading the box

Filter packaging is designed for a shopper standing in an aisle, and almost every number on it is measured in conditions you will never reproduce.

• “Suitable for tanks up to X litres.” Unregulated, and calculated for a light tropical stocking. Many keepers halve it for goldfish, as a rule of thumb.
• Litres per hour. Measured with an empty filter, no hoses and no height to lift against. A packed canister sitting in a cabinet under a tall tank can deliver half its headline figure, and it delivers less every week as the media loads.
• Head height. Usually printed in small type, and it is the number that tells you whether the pump can actually reach the rim of your tank. A canister on the floor under a 60 cm tank is lifting further than most people assume.
• Media volume. The figure that matters most, and the one least often printed — which is itself informative. A manufacturer who prints the litres of media a filter holds is usually proud of it.
• “Includes filter media.” Sometimes real ceramic and sponge; often a branded cartridge you are expected to buy again every month. That difference is a large hidden cost in a filter, and filter media explains why the cartridge is also where your bacteria live.
• Wattage. A filter runs continuously for a decade. The gap between a 10 W and a 35 W pump is small on any one day and not small over ten years, which what keeping goldfish costs puts in proportion.

What is not on the box at all is whether spares will still be sold in five years. Impellers, O-rings and baskets are what keep a good canister running into its second decade, and a discontinued model with no spares is a filter with a fixed lifespan whatever it is made of.

The filter is not the whole purchase

Budget for the filter and you have budgeted for most of it. These are the things people discover they need in the first fortnight.

• Biological media, if the filter ships with a cartridge. Ceramic rings or a matten-style sponge, bought once, replace a consumable you would otherwise buy monthly.
• A spray bar or flow diffuser, for any tank holding fancies. Stock nozzles produce a jet, and a ranchu pinned against the glass by it is a fish that eats badly and hides.
• A pre-filter sponge over the intake. It keeps fins, fry and gravel out of the impeller, and it takes the coarse load off the main media so the filter needs opening less often.
• A spare impeller and O-ring set. The two parts that fail, both cheap, both maddening to wait for while a tank sits unfiltered.
• A bucket that has never seen detergent, reserved for the tank. Cleaning a goldfish tank covers why that matters more than it sounds.
• An air pump and sponge filter, eventually, whether or not you buy them now. They are the cheapest insurance on the tank, and power cuts and emergencies explains what they buy you.

One large filter or two smaller ones

A question worth settling before you buy rather than after.

Two filters are more robust. If one fails, the tank is not unfiltered while you wait for a part. You can clean one this month and the other next month, which means you never disturb the whole colony at once. And a spare colonised filter is what turns a bucket into a quarantine tank the same afternoon a new fish arrives, or a hospital tank the afternoon one is needed.

One filter is quieter, cheaper and tidier, holds more media for the money, and is one thing to service rather than two.

What we would actually do: one properly sized filter, plus a sponge filter on an air pump. That combination costs barely more than the filter alone, adds a second independent colony, adds aeration — which the aeration guide argues matters more in a warm goldfish tank than most keepers expect — and gives you a portable cycled filter that can be lifted into another tank in seconds.

Buying second-hand

Canister filters hold their value badly and their usefulness well, which makes the used market the best value anywhere on this page.

What to check: that the motor head runs without grinding, that the body has no cracks around the hose sockets, that the taps still seal, and that the model is current enough for spares to exist.

What to assume you are replacing: the impeller and its shaft, the main O-ring, and often the hoses, which go stiff and stop sealing. Budget for those before you decide it is a bargain.

What not to expect: living bacteria. Dry media has probably died, and damp media may keep some bacteria alive for a few hours, but there is no source for a cut-off, so a used filter is cycled exactly as a new one is, and cycling a goldfish aquarium applies unchanged.

The disease question. A filter from a tank that lost fish is not a filter to put straight on yours. Strip it completely, discard the media, and either let every part dry hard for a fortnight or disinfect it, which many keepers do with a dilute bleach solution rinsed exhaustively and dechlorinated (no sourced strength is given here). The Merck Veterinary Manual advises disinfecting equipment and storing it dry, and drying is the simpler route.

When a new filter is not the answer

Three situations where the filter is not the thing to buy, and all three are common.

The tank is too small. Filtration handles ammonia; it does not create swimming room, and it dilutes nothing. If the tank is undersized, how many goldfish you can keep is the page to read instead of this one.

The filter is not cycled yet. A new filter on a new tank reads ammonia for weeks no matter how good it is. Buying a second one does not shorten that.

The filter is clogged, not undersized. Flow that has fallen off over months is often media that needs rinsing in old tank water, or an impeller wrapped in plant debris. Open it before you replace it. The filtration guide covers which media to rinse and which to leave alone, and cleaning a goldfish tank has the routine it belongs in.

Our picks explained

Fluval 407: best overall

This is the canister we would put under a tank of up to about 150–200 litres with two or three fancies: a 4.2 litre media basket for their waste, widely reported as quiet, and valves that make disconnecting for cleaning simple. Fluval rates it for tanks of 150 to 500 litres, but its filter circulation is 930 litres an hour, which is 3.1 to 4.7 turnovers an hour on 200 to 300 litres, below the 5–10 target above; the 1,450 litres an hour on the box is the pump alone, with no media in the way. Budget for the spray bar, because the stock nozzle produces a jet fancies do not appreciate. For a single fish in 100 litres, a hang-on-back does the job for less.

AquaClear 70: best value

The AquaClear’s virtue is its basket: big, open, and happy with whatever media you choose, so the bacteria colony is never thrown away with a cartridge. The flow lever recirculates water through the media rather than throttling the pump, which is why it suits fancies. It is not silent when the water level drops, and it needs a clean impeller to restart after a power cut.

Fluval FX4: best for large tanks

Adult commons and comets are 25–35 cm fish, and a 280-litre tank of them produces a lot of waste. This is a canister goldfish keepers often name for that job, and its edge is flow, rating and self-priming rather than media: Fluval rates it for tanks up to about 1,000 litres and its pump at 2,650 litres an hour, while its biological media volume is about 3.9 litres. It primes itself and has a utility valve that drains water during changes. It is heavy, needs cabinet space, and its output must be diffused through a spray bar around fancies.

Aquarium Co-Op Large Sponge Filter: best for fancy goldfish

For a ranchu, bubble eye or celestial, a sponge filter is often the best primary filter. The coarse foam holds a large bacteria population, the flow is a gentle upward column, and the air stream adds oxygen, as our aeration guide explains. The trade-off is polish: without fine mechanical media the water may look less crystalline, and you need a separate air pump.

Seachem Tidal 75: best self-priming hang-on-back

The Tidal answers the two usual complaints about hang-on-backs. Its pump sits inside the tank, so it restarts on its own after power cuts, and its skimmer removes the oily film that well-fed goldfish tanks develop. The media basket is generous and comes with good biological media; the in-tank pump takes up space, and some keepers report a hum as the media clogs.

Eheim Classic 600 (2217): built to last

There is nothing clever about this canister, which is the point. It is a bucket of media with a reliable pump on top, and reported service lives run to ten years and beyond. Its moderate output suits fancies, and Eheim says every classic model comes with a spraybar. Cleaning means unpacking the media layers by hand, and priming without the optional double taps is awkward, so it rewards a patient owner.

What we would skip

Undergravel filters, because goldfish waste and constant digging clog the gravel bed, and gravel itself is a swallowing hazard. Cartridge filters sold with starter kits, which are rated for far smaller bioloads and throw away your bacteria with every cartridge. Any filter with no way to reduce flow in a tank of fancies. And any filter marketed as making water changes unnecessary; none does, as our guide to whether goldfish need a filter explains.

Buy a filter rated above your tank, give it a month to cycle, and keep a second filter or sponge running if you can. Goldfish forgive many things, but not a filter that stops.