How many GPH should my aquarium filter be?

A common rule of thumb is to turn over the tank volume about 4 times per hour. So a 20-gallon tank wants roughly 80 GPH, a 55-gallon tank about 220 GPH. Because manufacturers' rated GPH is measured with no media or head height, choose a filter rated higher than the target to land near 4x in real use.

Can a filter be too powerful for a tank?

Yes for flow-sensitive fish like bettas and fancy goldfish, strong current is stressful. You can over-filter for biological capacity but reduce the current with a spray bar, baffle, or by aiming the outflow at the glass.

Aquarium Filter Size Calculator

Find the filter flow rate your tank needs. Enter your tank volume and we'll apply the standard 4× turnover rule of thumb.

Tank size (gallons)

Turnover target (times per hour)

Note on rated vs real flow: Manufacturers measure GPH with an empty filter and no head height. Once you add media and lift the water up to the tank, real flow is often 30–50% lower. That's why we suggest buying a filter rated higher than your target.

How filter sizing works

The goal of a filter is to cycle your whole tank through the media often enough to keep the water clear and the beneficial bacteria fed. The standard hobby guideline is roughly 4× the tank volume per hour:

Tank	4× target (real GPH)	Buy rated ~
10 gallon	40 GPH	60–80 GPH
20 gallon	80 GPH	110–160 GPH
29 gallon	116 GPH	150–200 GPH
55 gallon	220 GPH	300–350 GPH
75 gallon	300 GPH	400–450 GPH

Messy fish (goldfish, cichlids, large plecos) do better at 6–10×. Flow-sensitive fish (bettas, fancy goldfish) prefer the lower end or a baffled outflow.

Frequently asked questions

Should I count rated GPH or real GPH?

Plan around real (in-tank) flow. Because rated figures are optimistic, buy a filter rated above your target so it lands near the goal once it has media and head height to fight.

Is more filtration always better?

For biological capacity, more is generally safer. For current, no — some fish dislike strong flow. You can dial down current with a spray bar or baffle without losing filtration.

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