What flow rate filter do I need?

Aim for a filter that turns over your tank volume at least 4 times per hour, ideally around 6 times, and up to 10 times for heavily stocked or messy fish. For a 200-litre tank that's roughly 800–1,200 L/h. Enter your volume above to see the exact range.

What does turnover mean?

Turnover is how many times per hour the filter could cycle the entire tank: flow rate divided by tank volume. A 1,000 L/h filter on a 200-litre tank is 5× turnover. It's a better guide than raw flow because it accounts for tank size.

Can a filter be too strong?

Yes for some fish. Calm-water species like bettas and many gouramis dislike strong current, and turnover much above 10× can blow them around. You can fit a spray bar or baffle to spread the flow, but it's better to match the filter to the tank.

Why is my real flow lower than the rating?

Manufacturers measure flow with an empty filter and no lift. Once it's packed with media and pumping water up to the tank, real output is commonly 25–40% lower. Size up slightly, or choose a filter rated comfortably above your minimum, to allow for this.

Should I add up multiple filters?

Yes — if you run two filters, add their flow rates together for total turnover. Splitting filtration across two units also adds redundancy and lets you clean one at a time without disturbing the whole biological colony.

Free aquarium calculator

Aquarium Filter & Flow Calculator

Check whether your filter is the right size. Enter your tank volume and the filter's flow rate to see its turnover, and get a recommended flow range from lightly stocked to heavily stocked — in GPH or LPH.

Tank volume

The water volume your filter has to circulate.

Filter flow rate

Printed on the filter's box or specs. Leave blank to just see the recommended range.

Filter assessment

Turnover

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Recommended flow Minimum (4×) — Ideal (6×) — Heavy stock (10×) —

Results are estimates. Rated flow is measured with no media or head height; a filter full of media pumping up to a tank delivers less. Aim for the ideal range and prioritise good biological media over raw flow.

How the filter calculator works

Enter your tank's water volume and, if you have it, the flow rate of the filter you're considering. The calculator works out the turnover — how many times per hour the filter cycles the whole tank — and shows a recommended flow range, from the 4× minimum up to 10× for a heavily stocked tank. It converts between gallons per hour and litres per hour automatically.

Turnover, not just flow

A filter's flow rating only means something relative to your tank size. The useful figure is turnover: flow rate divided by tank volume. A common guideline is at least 4× the tank volume per hour for a lightly stocked or planted tank, around 6× for a typical community, and up to 10× for heavily stocked or messy fish such as goldfish and large cichlids. Beyond about 10× the current can be too strong for calm-water species.

Worked example

A 200-litre community tank wants roughly 800 L/h at the 4× minimum, about 1,200 L/h at the ideal 6×, and up to 2,000 L/h if heavily stocked. A filter rated at 1,000 L/h gives 5× turnover — comfortably in range. Remember the rated figure is measured empty; loaded with media and lifting water to the tank, real flow is often 25–40% lower.

GPH and LPH

Filters are rated in gallons per hour (GPH) in the US and litres per hour (LPH) elsewhere. One US gallon is 3.785 litres, so a 300 GPH filter moves about 1,136 L/h. Switch the unit toggle to see your recommended range in whichever you shop in.

Frequently asked questions

What flow rate filter do I need?: Aim for a filter that turns over your tank volume at least 4 times per hour, ideally around 6 times, and up to 10 times for heavily stocked or messy fish. For a 200-litre tank that's roughly 800–1,200 L/h. Enter your volume above to see the exact range.
What does turnover mean?: Turnover is how many times per hour the filter could cycle the entire tank: flow rate divided by tank volume. A 1,000 L/h filter on a 200-litre tank is 5× turnover. It's a better guide than raw flow because it accounts for tank size.
Can a filter be too strong?: Yes for some fish. Calm-water species like bettas and many gouramis dislike strong current, and turnover much above 10× can blow them around. You can fit a spray bar or baffle to spread the flow, but it's better to match the filter to the tank.
Why is my real flow lower than the rating?: Manufacturers measure flow with an empty filter and no lift. Once it's packed with media and pumping water up to the tank, real output is commonly 25–40% lower. Size up slightly, or choose a filter rated comfortably above your minimum, to allow for this.
Should I add up multiple filters?: Yes — if you run two filters, add their flow rates together for total turnover. Splitting filtration across two units also adds redundancy and lets you clean one at a time without disturbing the whole biological colony.