How to size an aquarium filter

Multiply your tank volume in gallons by 6, then divide by 0.7. That is the minimum RATED GPH to shop for. The multiplication is the turnover your tank needs; the division accounts for the roughly 30% of rated flow a filter loses to media and head height in service. A 10 gallon tank needs 85 rated GPH, not 60.

Filters are sold on a tank-size label, and that label is the least informative number on the packaging because it does not state what stocking level it assumes. Two filters can both claim "up to 20 gallons" while one moves half again as much water as the other. The calculation below replaces the label entirely.

Step one: turnover

Turnover is how many times per hour the filter moves the tank's whole volume. It is the meaningful measure because it scales with the thing that matters — how much waste is being produced per unit of water.

• 4 to 6 times per hour — a lightly stocked or planted tank. Planted tanks sit at the lower end deliberately: heavy surface agitation drives off the CO2 plants want.
• 6 to 8 times per hour — a typical community tank. This is what most beginner tanks become within six months, whatever the plan was.
• 8 times or more — heavily stocked, or messy feeders like goldfish and cichlids.

We size to 6x as the working default, because it matches where a first tank realistically ends up rather than where it starts. If you know your tank will stay lightly stocked, 4 to 5 is defensible and will save you money.

Step two: the 30% you never get

This is the step almost every filter recommendation online omits, and it is the reason so many beginner tanks are quietly under-filtered.

A filter's rated GPH is measured under laboratory conditions: brand-new media with no debris in it, and no height to lift the water. Your filter runs with a sponge, a bag of ceramic and a carbon insert in the flow path, and it lifts water up an intake tube and back over the tank rim. Both cost throughput. Real-world flow commonly runs 25 to 40% below the rating even when clean, and it falls further as the media loads between cleanings.

We assume a 30% loss, so we divide the required real flow by 0.7. Nothing about that figure is exotic — it is simply the correction that turns a laboratory number into a usable one.

Worked examples

1. A 10 gallon community tank. 10 x 6 = 60 GPH real flow needed. 60 / 0.7 = 86, round to 85 GPH rated minimum. A filter rated 60 GPH would be delivering about 42 in service — barely four turnovers.
2. A 20 gallon community tank. 20 x 6 = 120. 120 / 0.7 = 171, so 170 GPH rated minimum. This is why almost nothing labeled for 20 gallons clears it.
3. A 20 gallon planted tank, lightly stocked. 20 x 5 = 100. 100 / 0.7 = 143, so 145 GPH rated. The AquaClear 30 at 150 clears this and not the previous line — same tank, different stocking, different answer.
4. A 5 gallon betta tank. 5 x 6 = 30. 30 / 0.7 = 43, so 45 GPH rated. Almost any filter clears this, which is why a betta tank is a flow-control problem rather than a flow-capacity problem.
5. A 29 gallon community tank. 29 x 6 = 174. 174 / 0.7 = 249, so 250 GPH rated. Now you are into two hang-on-backs or a canister.

The table, for every common size

What the number does not tell you

Turnover is a proxy, and it is worth being clear about what it is standing in for, because that determines when you can safely ignore it.

What a tank actually needs is enough colonized biological surface area to process its ammonia load, plus enough water movement to carry debris to the mechanical stage. Flow correlates with both, which is why the number works. But a filter with a large media volume and modest flow can out-perform a high-flow filter with a thin cartridge, because the first has far more surface for bacteria to live on.

This is why adding a sponge filter is such an effective fix for a slightly undersized hang-on-back. It contributes almost nothing you can measure in GPH and a great deal of the thing the GPH figure was standing in for.

Can a filter be too strong?

For biological purposes, essentially no. Over-filtering a freshwater tank does not harm the nitrogen cycle and gives you tolerance for the overfeeding that happens in every real household.

The real limit is current, not filtration. Bettas, shrimp, fry and long-finned fish are pushed around by strong flow, and a tank where fish hide permanently in the one still corner is over-flowed regardless of what the arithmetic says. The fix is never a smaller filter — it is a baffle, a spray bar, a pre-filter sponge on the outflow, or aiming the outflow at the glass. All of those cost close to nothing and none of them reduce filtration.

Now check a real filter

Take the box in front of you, find the rated GPH — not the tank-size range, the GPH — and compare it to your number from the table. If the box gives no GPH figure at all, which is common for bundled kit filters and universal for sponge filters, then you cannot do this check and neither can anyone else. That is worth knowing rather than guessing around.

Our picks with the arithmetic already applied: best aquarium filters, best filter for a 10 gallon tank, and best filter for a 20 gallon tank.