Aquarium filter flow rate calculator
Turnover — how many times per hour the filter moves the tank’s volume — is the number that matters. Here is the flow to aim for and the filter rating to buy.
Turnover targets
| Tank type | Turnover per hour |
|---|---|
| Betta, shrimp or lightly stocked | 3–5× |
| Community tropical | 4–6× |
| Planted tank | 5–8× |
| Goldfish, cichlids or heavy feeders | 8–10× |
| Hillstream or river species | 10–15× |
| Reef / marine (total circulation) | 20–40× |
Rated flow vs real flow
Manufacturers measure flow with an empty filter and no tubing. Once a canister is packed with media and has to push water up to the tank, or a hang-on-back filter's sponge starts to clog, real flow is commonly 30–50% lower. The calculator assumes 30% loss, so a tank that needs 200 GPH of real flow wants a filter rated around 285 GPH.
Can there be too much flow?
Yes. Bettas, fancy goldfish and small fish such as chili rasboras struggle in strong current. If a filter is rated far above your needs, turn down its flow control, point the output at a wall or add a pre-filter sponge. In planted tanks, aim for gentle movement across the whole tank rather than one strong jet. Read the filter flow rate guide.
FAQ
What GPH filter do I need for a 55-gallon tank?
About 200–300 GPH of real flow for a community tank, so a filter rated roughly 300–450 GPH. Goldfish and cichlid tanks need about twice that.
Is 10× turnover too much?
Not for goldfish, cichlids or river species, which produce a lot of waste or like current. It is too much for bettas and slow-moving fish.
How do I convert GPH to liters per hour?
Multiply GPH by 3.785. A 300 GPH filter moves about 1,136 L/h.