What does rated GPH vs real GPH mean?

Rated GPH is the flow a filter achieves empty and with no lift. Real GPH is what you get after adding filter media and pumping the water up into the tank, which can be 30-50% lower. Always plan around real flow.

What size filter do I need for my aquarium?

Short answer: choose a filter that turns over your tank volume about 4 times per hour. That means roughly 40 GPH for a 10-gallon, 80 GPH for a 20-gallon, ~116 GPH for a 29-gallon, and ~220 GPH for a 55-gallon. Because manufacturers' rated GPH is measured with no media and no lift, buy a filter rated about 1.5× higher than that target so real flow lands near 4×.

Open the filter size calculator

The 4× turnover rule

A filter has two jobs: physically clearing debris and housing the beneficial bacteria that process fish waste. To do both, it needs to cycle the entire tank through its media often enough. The common guideline is four tank volumes per hour. Heavily stocked tanks or messy fish want more (6–10×); calm, lightly stocked, or flow-sensitive setups want less.

Rated GPH vs real-world GPH

This is where most people under-buy. The GPH printed on the box is measured with the filter empty and pumping water straight out — no sponge, no carbon, no lifting the water up into the tank. In reality:

Filter media adds resistance.
Head height (the lift back into the tank) cuts flow further.
Over time, a dirty filter slows down even more.

Expect 30–50% less than the rated number in everyday use. That's why our calculator suggests a rated GPH about 1.5× your real-flow target.

Filter size by tank

Tank	4× target (real GPH)	Buy rated ~
10 gallon	40 GPH	60–80 GPH
20 gallon	80 GPH	110–160 GPH
29 gallon	116 GPH	150–200 GPH
40 gallon	160 GPH	240–300 GPH
55 gallon	220 GPH	300–350 GPH
75 gallon	300 GPH	400–450 GPH

Hang-on-back vs canister vs sponge

Hang-on-back (HOB): simplest and cheapest, great up to ~40–55 gallons.
Canister: more media and flexibility, best for larger or planted tanks.
Sponge filter: gentle, cheap, ideal for nano tanks, fry, and bettas; pair with an air pump.

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Filters by GPH tier: match the "buy rated" column above to a reliable HOB or canister.

The 4× rule is a starting point, not a law. Lightly stocked, heavily planted tanks can do fine with less; goldfish and cichlid tanks usually want more. Aim for stable, clear water and adjust based on what your tank actually does.

Frequently asked questions

Can a filter be too strong?

For biology, rarely. For current, yes — bettas and fancy goldfish dislike strong flow. Use a spray bar or baffle to soften it without losing filtration.

Should I run two filters?

On larger tanks it's a good idea: more total media and a backup if one fails. It also makes maintenance easier since you can clean one at a time without crashing your bacteria.

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