Which kind of filter suits this tank
- A large hang-on-back filter, or two smaller ones, covers this size and gives you a backup if one stops.
- A canister filter is quieter and holds more media if the tank is heavily stocked.
How filter flow is calculated
Filters are sized by turnover: how many times an hour the whole volume of the tank passes through the filter.
Different tanks need different turnover:
| Tank | Turnovers per hour | Why |
|---|---|---|
| Betta, shrimp | 2–5 | Weak swimmers and tiny animals need gentle flow |
| Community tropical | 4–6 | The standard range for a normally stocked tank |
| Planted | 4–8 | Enough to carry CO₂ and nutrients to every leaf |
| Goldfish, cichlids, big fish | 6–10 | Heavy feeders that produce a lot of waste |
| Marine, fish only | 6–10 | High oxygen demand |
| Reef | 10–20 or more | Total water movement, mostly from wavemakers |
A lightly stocked tank can sit about 15% below these ranges; a heavily stocked one needs about 25% more.
Worked example. A 55-gallon community tank at 4–6 turnovers needs 220–330 GPH (about 830–1,250 L/h).
Rated flow is not real flow
The figure on the box is measured with the filter empty and the pump lifting water no height at all. Fill the baskets with media, raise the outlet to the top of the tank, let the sponges collect a week of dirt, and real flow is typically a third to a half lower. The ranges above are box ratings chosen with that loss in mind, so you can compare them directly with what manufacturers print.
Flow is not the whole filter
Turnover moves the water, but the cleaning is done by bacteria living on the filter media. Two filters with the same flow can differ greatly in how much media they hold. When choosing between filters with a similar rating, pick the one with the larger media volume, and never replace all the media at once: rinse it in old tank water instead, so the bacteria survive. Our guide to the nitrogen cycle explains why.
Common mistakes
- Buying by "suitable for up to X gallons". That label assumes a light fish load. Compare flow ratings instead.
- Washing media under the tap. Chlorine kills the bacteria that do the filtering. Rinse in a bucket of tank water.
- Replacing cartridges every month as the box suggests. That throws the bacteria away. Replace only when the media is falling apart, and never all at once.
- Too much flow for the fish. Bettas, gouramis and fancy goldfish struggle in strong current: baffle the outlet or choose a lower flow.
Questions and answers
How many GPH does my aquarium filter need?
Multiply your tank volume in gallons by 4 to 6 for a community tank, or 6 to 10 for goldfish, cichlids and other messy fish. A 30-gallon community tank needs a filter rated 120–180 GPH.
What is a good turnover rate for a fish tank?
Four to six times the tank volume an hour for most freshwater tanks. Bettas and shrimp do better at 2–5 times; reef tanks need 10–20 times or more in total water movement.
Can you over-filter a fish tank?
You cannot have too much biological filtration, but you can have too much current. If fish are pushed around or hide from the flow, spread the outlet with a spray bar or turn the flow down.
How do I convert GPH to liters per hour?
Multiply US gallons per hour by 3.785. A 200 GPH filter moves about 757 L/h. For UK gallons, multiply by 4.546.
Do I need two filters?
On tanks over about 75 gallons (285 L), two filters are a good idea: they share the load, and one keeps the tank alive if the other stops or is being cleaned.
Numbers for standard tanks
Have a standard tank? Its size, weight, heater and stocking are already worked out.
- 10 gallon tank
- 20 gallon long tank
- 29 gallon tank
- 40 gallon breeder tank
- 55 gallon tank
- 75 gallon tank
- 125 gallon tank
- All tank sizes
Related guides
- How to Set Up Your First Aquarium, Step by Step
- How Many Fish Can I Put in My Tank?
- The Aquarium Nitrogen Cycle, Explained
Sources
- Turnover ranges reflect long-standing practice among filter manufacturers and aquarium clubs; rated-flow losses vary by model.