What size filter for a 20 gallon tank?

On this calculator, a 20-gallon tank at the normal level is a 120 GPH rating. Light is 100. Heavy is 170.

What turnover is for

Turnover is a proxy for the two jobs filtration does: mechanical (pulling waste into media before it rots) and biological (housing the nitrifying bacteria that turn ammonia to nitrate). Rinse mechanical media in removed tank water. Chlorine in tap water kills the bacterial colony.

HOB vs sponge vs canister for a 20

• HOB (hang-on-back): the 20-gallon default — meets turnover, best mechanical cleaning, cheap media. Weakness: intake sucks up shrimp, fry, and long-finned bettas; add a prefilter sponge.
• Sponge: unbeatable biology-per-dollar and fry-safe, but weak mechanically. A large sponge + air pump is the right call for a shrimp tank or breeding tank; pair with a small HOB for a community.
• Canister: overkill for most 20s — the money is better spent on stock quality — but the answer if the tank is next to a bedroom (quieter) or you want weeks between maintenance.

Filter flow and your stocking points

Filter size and bioload are two halves of one budget. A 20-gallon normal setup has a capacity of 20 points (see how many fish in a 20 gallon). If nitrate climbs faster than the water-change schedule you measured, lower the bioload or switch the calculator to the heavy filter level. The nitrate math is in the calculator. For a longer active fish, see what size tank zebra danios need.

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Media: the three layers and what each is for

Inside any filter, three jobs stack in order: mechanical (sponge or floss, which catches debris), biological (the bacteria housing; do not throw it out), and chemical (optional polish). The common mistake is replacing all of the media when a cartridge looks tired, which throws away the bacterial colony and restarts the cycle. Keep a piece of permanent bio-media in the box so a cartridge swap does not do that.

Setup order for a new 20: rinse the media in tap water to clear dust before any bacteria are present, assemble, dechlorinate the tank, and run a fishless cycle. Add fish only once ammonia and nitrite are gone. The clocks, and the ammonia amount in the Merck chapter, are on how to fix cloudy water in a new tank. A filter on an uncycled tank is just a pump.

What this calculator returns

Section added October 6, 2026. Built by Jeremy Panasuk. The numbers below are the filter tab on the calculator. Bioload points, watts per gallon, and filter turnover here are planning estimates for a cycled freshwater tank, not a published standard.

The function picks turnovers by level: 4 an hour for light, 5 for normal, and 7 for heavy. Ideal flow is gallons times those turnovers. The rated gallons per hour it prints is ceil(ideal × 1.2 / 5) × 5, which is a 20% step rounded up to the next 5. That 20% is this site's allowance. It is not a measured loss from a manufacturer's test.

Normal: 20 × 5 = 100, then ceil(100 × 1.2 / 5) × 5 = ceil(24) × 5 = 120. Light: 20 × 4 = 80, ceil(80 × 1.2 / 5) × 5 = ceil(19.2) × 5 = 100. Heavy: 20 × 7 = 140, ceil(140 × 1.2 / 5) × 5 = ceil(33.6) × 5 = 170.

Twenty-nine gallons at 5 turnovers is 145 gallons per hour ideal, and ceil(145 × 1.2 / 5) × 5 = 175. Fifty-five gallons at 5 turnovers is 275 ideal, and ceil(275 × 1.2 / 5) × 5 = 330. Stocking and heater for those same volumes are on how many fish in a 10, 29, or 55 gallon tank.

The common mistake is buying the box that says it is for a 20-gallon tank and treating that sentence as the function above. A box rating and this 120 gallon-per-hour result are not the same measurement. No manufacturer manual used for this section states how much of the box rating remains once media is loaded, so this page does not state one.

The rating does not grow bacteria, and it does not know whether your fish want calm water. If fish are dying, start with how to treat sick fish and a veterinarian, not a larger pump. Once nitrate is what you are managing, the percent comes from how often to change aquarium water.

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.

Sources

The gallons-per-hour figures in this section are this site's filter function. They are planning estimates, not a published filter standard.

FAQ

What GPH filter for a 20 gallon tank?

On this calculator, 20 gallons at the normal level is 5 turnovers an hour, an ideal flow of 100 gallons per hour, and a rating of 120. Light is 100. Heavy is 170. Those figures are this site's function, not a measured loss from the box rating.

Can a filter be too strong for fish?

A strong current can push long-finned fish around. This page does not give a turnover multiple for that, because no source used here states one. Baffle a strong hang-on-back if the fish want calm water.

How often should I clean the filter?

Rinse mechanical media in removed tank water, and replace it only when it is falling apart. Never rinse it under tap water, and never replace all the media at once. The bacterial colony lives in the media.

More: 20-gallon stocking lists, bioload points explained, heater wattage by ΔT, and the bioload calculator.