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
Common questions
What GPH filter do I need for a 20-gallon tank?
Aim for about 4 to 6 times the tank volume per hour, so roughly 80 to 120 GPH for a 20-gallon. Hang-on-back filters lose flow as media clogs, so the higher end gives useful headroom.
Can an aquarium filter be too powerful?
For most tanks more flow is fine, but bettas, shrimp and fry dislike strong current. If the flow is too much, baffle the outflow or turn it down rather than undersizing the filter.
Does the GPH on the box match real flow?
Rarely. Rated GPH is measured with an empty filter and no lift height; real flow drops once media is in and as it clogs. Size up and rinse the media regularly.
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