What GPH filter do I need for my aquarium?

Aim for a real-world flow of about 4 to 6 times your tank volume per hour. A 10-gallon wants roughly 50-75 GPH, a 20-gallon 100-150 GPH, a 30-gallon 150-200 GPH, a 55-gallon 275-350 GPH, and a 75-gallon 375-450 GPH. Increase toward 6-10x for goldfish, cichlids, or heavily stocked tanks. Because rated GPH is optimistic, buy a filter rated about 1.5x your target.

Is a canister or hang-on-back filter better?

Hang-on-back filters are cheaper and simpler and work well up to about 55 gallons. Canister filters hold far more media, run quieter, and are the better choice for tanks over 55 gallons, planted tanks, and messy fish. Many keepers run a canister plus a small HOB for redundancy on large tanks.

Aquarium filter size chart: GPH and the right filter for every tank

Short answer: size your filter for a real-world flow of about 4–6× your tank volume per hour (push to 6–10× for goldfish, cichlids, or heavily stocked tanks). That means roughly 50–75 GPH for a 10-gallon, 100–150 GPH for a 20-gallon, 150–200 GPH for a 30-gallon, 275–350 GPH for a 55-gallon, and 375–450 GPH for a 75-gallon. Because rated GPH is measured with no media and no lift, buy a filter rated about 1.5× higher than that target.

Open the filter size calculator

The filter size chart

Find your tank size, read across to the GPH you want, then pick a filter. The hang-on-back (HOB) and canister suggestions below are each sized using the manufacturer's own tank-size rating, so they comfortably cover the row. Numbers in the GPH columns are simply your tank volume multiplied by the turnover rule, not a product claim.

Tank size	Target GPH (4–6×)	Heavy stock (6–10×)	Suggested HOB filter	Suggested canister filter
5 gallon	25–50	up to 75	AquaClear 20	sponge filter (or none)
10 gallon	50–75	up to 100	AquaClear 20 / Seachem Tidal 35	—
20 gallon	100–150	up to 200	Seachem Tidal 55 / AquaClear 50	Fluval 107
30 gallon	150–200	up to 300	AquaClear 70 / Seachem Tidal 75	Fluval 207
40 gallon	200–280	up to 400	AquaClear 70	Fluval 207 / Eheim Classic 250
55 gallon	275–350	up to 550	AquaClear 110	Fluval 307 / Oase BioMaster 350
75 gallon	375–450	up to 750	AquaClear 110 (×2)	Fluval 407 / Oase BioMaster 600
100 gallon	500–600	up to 1000	—	Fluval FX4 / Eheim Classic 600
125 gallon	625–750	up to 1250	—	Fluval FX6 / Oase BioMaster 850

Not sure of your exact volume, or running an odd tank size? The filter size calculator takes any gallon or liter figure and returns your target and rated GPH instantly.

The 4–6× turnover rule, in one minute

A filter clears debris and houses the bacteria that process fish waste. To do both, it has to cycle the whole tank through its media often enough. Four to six tank volumes per hour suits most community tanks. Messy or heavily stocked tanks — goldfish, cichlids, plecos — want more, in the 6–10× range. Calm, lightly stocked, or flow-sensitive setups (bettas, fancy goldfish) want less, and benefit from a baffle or spray bar to soften the current.

Why you should buy bigger than the GPH math says

This is where most people under-buy. The GPH printed on the box is measured with the filter empty and pumping water straight out, with no sponge, no carbon, and no lifting the water back up into the tank. In real use you lose flow to:

Filter media, which adds resistance.
Head height, the lift back into the tank.
A dirty filter, which slows further between cleanings.

Expect 30–50% less than the rated number day to day. That is why the chart's suggested models sit a step above the raw GPH target, and why the calculator recommends a rated GPH about 1.5× your real-flow goal.

HOB vs canister, by tank size

5–55 gallon → hang-on-back. Cheapest, simplest, easiest to maintain. AquaClear and Seachem Tidal are the two most consistently recommended lines among hobbyists.
55 gallon and up → canister. Far more media, quieter, and tidier plumbing. The Fluval 07-series scales cleanly from a 20-gallon (107) to a 100-gallon (407); the FX line covers the biggest tanks.
Planted or cichlid tanks → premium canister. Oase BioMaster (with its pre-filter module) and Eheim Classic are the long-haul picks named most often for these setups.

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The turnover rule is a starting point, not a law. Heavily planted, lightly stocked tanks can do well with less flow; goldfish and cichlid tanks usually want more. Aim for stable, clear water and adjust to what your tank actually does. Model suggestions reflect each maker's published tank-size rating and common hobbyist consensus, not a guarantee of fit for every setup.

Frequently asked questions

What is the best filter for a 30 gallon tank?

Target 150–200 GPH. An AquaClear 70 or Seachem Tidal 75 hang-on-back covers it simply; a Fluval 207 canister adds more media and quieter flow. Goldfish or cichlid 30-gallons should push toward the higher end.

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 is 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 colony.

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