Filter GPH calculator
How much filter flow does your tank need? Set the tank size, stocking level and filter type for the rated GPH to shop for — and which of our reviewed filters clear it.
Heavily-planted tanks run cleaner (dose toward light–moderate); goldfish and cichlids are messy (treat them as heavy). Manufacturer GPH is measured with an empty filter, so this sizes up for real flow after media.
The turnover rule
Filter capacity is measured as turnover: how many times per hour the filter cycles the whole tank volume. A rough working range is 4× for a lightly stocked or planted tank, 6× for a typical community tank, and 8–10× for a heavy bioload like goldfish or cichlids. Multiply your tank volume by that number and you have a GPH target.
Why you should buy bigger than the target
Manufacturers measure GPH with an empty filter. Once you add media and it starts to trap gunk, real flow drops — often by a third or more. So a filter rated right at your target will run under it in practice. Size up: aim for a rating around 1.5× your turnover target, and turn the flow down if it's too strong (adjustable models make this easy).
Turnover isn't the whole story
Flow gets water to the media; it's the biological media that actually keeps fish alive by hosting the bacteria that convert ammonia to nitrite to nitrate. A filter with a big, refillable media basket beats a higher-GPH one with a thin proprietary cartridge. We cover that trade-off — and the recurring cartridge cost — in the best filter for a 20-gallon tank under $50 guide and the full aquarium filters rankings.
Source
- Filter turnover / GPH sizing for aquariums — FishLab
- The nitrogen cycle — why biological media matters more than raw flow — Aquarium Science
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