What size filter for a 20 gallon tank?

4–6 turnovers per hour → 80–120 GPH. One formula, one chart, and the derating that makes "rated for 20 gallons" mean less than it says.

The turnover formula

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). Both scale with flow through the media, which is why the formula works.

The chart by tank size

The derating nobody mentions

Rated GPH is measured with clean media and no restrictions. In a real tank — media clogged with debris, intake clogged with mulm, spray bar diffusing output — delivered flow lands around half the rating. A "120 GPH" HOB is often pushing 60. This is the actual reason the advice is "size up, never down," and why rinsing mechanical media monthly (in removed tank water, never tap — chlorine kills the bacterial colony) is part of filtration, not an optional chore.

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 stocked to ~20 bioload points (see how many fish in a 20 gallon) at 6× turnover has real headroom; the same stock on a 4× undersized filter shows it in nitrate climbing between changes. If nitrates rise past ~20 ppm faster than your weekly schedule, either drop a point class or move up the turnover band — the nitrate-balance math is in the calculator.

Media: the three layers and what each is for

Inside any filter, three jobs stack in order: mechanical (sponge/floss — catches debris, rinse monthly), biological (ceramic rings, bio-home, lava rock — the bacteria housing, clean never), and chemical (carbon, purigen — optional polish, replace on a schedule). The single most common 20-gallon mistake is replacing all three layers when the "cartridge" looks tired, which throws away the entire bacterial colony and restarts the cycle you spent six weeks building. Cartridge-style HOBs make this trap easy to fall into; the fix is to keep a piece of permanent bio-media in the box regardless of what the cartridge schedule says, so a cartridge swap never means a cycle crash.

Setup order for a new 20: rinse the media in tap water to clear dust (before the bacteria arrive, chlorine is harmless), assemble, dechlorinate the tank, and start the fishless cycle — dosing ammonia to 1–2 ppm and testing until ammonia and nitrite both read 0 within 24 hours. Only then does the turnover number start protecting your bioload budget; a filter on an uncycled tank is just a pump.

FAQ

What GPH filter for a 20 gallon tank?

80–120 GPH rated — 4–6 turnovers per hour. Buy the 6× end if the tank will run heavily stocked, and remember delivered flow is roughly half the box rating once media loads.

Can a filter be too strong for fish?

Yes — flow, not filtration. Bettas, guppies, and long-finned species get blown around above roughly 8–10× turnover; planted tanks also gas off CO2 at high flow. Stay in the 4–6× band and baffle a strong HOB's output with a sponge if your stock prefers calm water.

How often should I clean the filter?

Rinse mechanical media every 2–4 weeks in removed tank water, and replace only when it's falling apart — never under tap water and never all the media at once. The bacterial colony lives in the media; a fully replaced filter restarts the cycle.

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