What a filter actually does
A filter does three jobs at once. It provides mechanical filtration (trapping visible debris), biological filtration (housing the bacteria that turn toxic ammonia and nitrite into far safer nitrate), and optionally chemical filtration (removing dissolved odours, tannins and medication). The biological stage is the one that keeps fish alive — it is where your nitrogen cycle lives — so any filter you choose must have room for plenty of bio media.
Get the filter right and most other problems shrink. Get it wrong — undersized, or all mechanical and no biology — and you will fight cloudy water, ammonia spikes and stressed fish no matter what else you do. Two things decide whether a filter is right for your tank: how much water it moves per hour, and how much media it can hold. The rest is choosing the format that suits your setup and stocking.
The 4× turnover rule
Filters are rated by flow in litres per hour (L/h). The simple sizing rule is to aim for a filter that turns over roughly 4× your tank's volume every hour:
- A 60L tank wants around 240 L/h.
- A 75L tank wants around 300 L/h.
- A 200L tank wants around 800 L/h.
Manufacturer flow ratings are measured with an empty filter, so real-world flow drops once the box is packed with media and starts to clog. That is one more reason to build in headroom rather than buy exactly to spec. Head height matters too — external canisters sitting well below the tank push against gravity, so their effective flow is a little lower than the number on the box suggests.
Matching the type to your tank
- Canister (external) filters sit under the tank and hold the most media. They are the best choice for bigger and planted tanks where you want serious biological capacity and quiet, tidy plumbing. See our best external filter pick.
- Internal and hang-on-back (HOB) filters are simple, affordable and easy to service — a great fit for small and medium tanks. Our best internal filter round-up covers the strong options.
- Sponge filters, run off an air pump, give gentle flow and lots of bacterial surface. They are ideal for fry, shrimp and quarantine tanks where a powerful intake could injure tiny livestock.
Match the filter to your stocking
Turnover is the starting point, not the whole story. A lightly stocked, planted tank can sit comfortably at the lower end of the range, while messy or heavily stocked tanks — goldfish, big cichlids, a crowd of active fish — produce far more waste and benefit from extra capacity or a second filter. Always size for the fish you plan to keep, not just the ones in there on day one. Our filter hub breaks down capacity by tank size if you want to compare models directly.
Loading the media
Whatever type you choose, arrange the media so water passes through it in order:
- Mechanical first — coarse then fine sponge or floss to strip out debris before it clogs the rest.
- Biological next — ceramic rings, bio-balls or a porous matrix, the largest stage, where your bacteria colony lives.
- Chemical last and optional — activated carbon or resins to polish the water or pull out medication, replaced on a schedule.