How to choose an aquarium filter

The filter is the most important piece of equipment in the tank, and the most misunderstood. It is not really there to make the water look clear; it is there to keep your fish alive by housing the bacteria that process their waste. Here is how the four common types compare, how to size one, and what to put inside it.

Published 21 June 2026 · Last updated 21 June 2026 · 14 min read

What an aquarium filter actually does

A filter pulls water through some media and returns it to the tank. What that achieves comes down to three separate jobs, and most filters do all three at once:

Mechanical filtration traps physical debris: uneaten food, fish waste, and plant matter caught on a sponge or floss. This is the visible "dirt" you rinse out. It keeps the water clear but does nothing to make it safe on its own.

Biological filtration is the one that keeps fish alive. Beneficial bacteria colonise the filter media and convert toxic ammonia from fish waste into nitrite, then into far less toxic nitrate. This is the same nitrogen cycle you grow when you cycle a new tank, and the bulk of those bacteria live in the filter. Caring for your filter is really caring for your cycle. For what each of those readings means, see the water parameters guide.

Chemical filtration is optional. Media such as activated carbon or a resin like Seachem Purigen adsorb dissolved compounds, odours, tannins, or leftover medication. It is useful in specific situations rather than something a healthy tank needs running all the time.

The four types of filter

Four types cover almost every freshwater tank. Open the one you are weighing up. Each has a clear sweet spot, and none is best at everything.

Which filter for which tank

If you want a quick answer, match your tank to the table below, then read the type's card above for detail.

Many experienced keepers run two filters on one tank, for example a canister plus a sponge filter. It is not required, but the redundancy means that cleaning or replacing one filter never crashes the cycle, because a fully colonised second filter keeps working throughout.

Sizing a filter: flow and turnover

Filters are rated by flow in litres per hour (L/h). The useful number is turnover: how many times per hour the filter could move your whole tank's volume. Aim for roughly 4 to 6 times your tank volume per hour for a lightly stocked tank, and 6 to 10 times for a busy or messy one. A 100 litre tank therefore wants somewhere around 400 to 1000 L/h depending on how it is stocked.

The catch is that rated flow is optimistic. Media, hose length, the height the pump has to lift water, and a filter that is due for a clean all cut real-world flow, often by a third or more. So buy a filter rated above your target rather than exactly at it. Slightly oversized is a good thing: you can always reduce flow, and a larger filter holds more media and more bacteria.

Work from your tank's real water volume, not the figure on the box. A "100 litre" tank holds noticeably less once you account for substrate, decor, and the gap below the rim; the tank volume calculator estimates it from your dimensions. If you enter your filter's rated flow into the stocking calculator, it will also show your turnover.

One caveat on flow: some fish dislike strong current, including bettas, gouramis, and other slow, long-finned species. If you keep them, choose a filter with adjustable flow, or baffle the outlet with a spray bar or a piece of sponge to soften it.

Filter and heater are the two pieces of equipment you size to the tank. While you are at it, size the heater with the heater wattage calculator.

Filter media: what to use, what to skip

Media is whatever sits inside the filter and does the work. It comes in three kinds, matching the three jobs:

Mechanical media (sponges, filter floss or wool, pads) traps debris. Rinse it regularly in old tank water, and replace floss only when it starts to break down. It is cheap and does not hold the bulk of your bacteria.

Biological media (ceramic rings, sintered glass such as Seachem Matrix, or simply the sponges themselves) gives bacteria a high-surface-area home. This is the media you protect: never replace it all at once, and never rinse it under the tap.

Chemical media (activated carbon, or a rechargeable resin like Seachem Purigen) is optional. Carbon removes tannins, odours, and medication, so take it out while you are dosing a treatment. You do not need chemical media running permanently in a healthy tank; add it to solve a specific problem.

Never replace all the media at once. Because it holds the living colony, swapping everything in one go is like cleaning your filter media in tap water: it crashes the cycle. Stagger any replacement so an established colony always remains.

Mind the order. Water should pass through mechanical media first, so debris does not clog the biological media, then through biological media, with chemical media last. Most filters are arranged this way already; keep that order when you pack a canister.

Setting up and maintaining a filter

Run it before any fish arrive. Install the filter and run it for the whole of your cycling period. The bacteria grow on the media during cycling, so a filter is not "ready" simply because it switched on this morning.

Keep it running 24 hours a day. Never switch the filter off overnight or for long stretches. The bacteria need oxygenated flow, and a filter left off for more than an hour or two begins to die back, while a stopped canister can foul and release that waste when restarted.

Do not over-clean. A filter that looks dirty is usually just full of bacteria and working perfectly. Over-cleaning does more harm than leaving it alone. Let falling flow, not the calendar, tell you when it is time.

Priming and flow. Canisters and some hang-on-back filters need priming to start the siphon; follow the model's instructions. If flow does not return after a clean, re-prime it or check that the impeller is seated and free of debris. A sudden drop in flow is almost always a clogged intake or a dirty impeller, not a dead motor.

Australian buying notes

Buy Australian-certified equipment. Any filter you buy should be certified for Australian 240 V mains. Avoid uncertified equipment from overseas marketplaces for anything electrical that sits in or near water; the saving is not worth the risk. This is the same advice as in our setup guide.

Brands stocked here. Australian shops widely carry Aqua One (a local favourite with good value across every type), Eheim and Fluval (canisters), and AquaClear and Seachem Tidal (refillable hang-on-back filters). Aqua One spares and media are especially easy to find, which matters more than it sounds for long-term ownership.

Spares decide the real cost. Choose a brand whose impellers, sponges, and media are stocked locally. An obscure imported filter is useless the day its impeller wears out and no one carries the part.

Running cost is small. Filters draw very little power, from a few watts up to around 30 W, so running one 24 hours a day costs very little even at Australian electricity prices. The air pump for a sponge filter is just as cheap to run.

Where to buy. Petbarn, Petstock, and dedicated aquarium shops stock the common types and media. Specialist online retailers carry the wider range of canisters and replacement media.

Your filter is one part of the whole: see how to set up a new aquarium, how to cycle it so the filter grows its bacteria, and how to keep it healthy with regular water changes. To size your heater too, use the heater wattage calculator, and browse fish by tank size in the species directory.