Aquarium Filter Size Calculator
The right filter depends on your tank type. Enter your tank size below for a recommended flow rate in GPH (gallons per hour).
Calculate filter flow rate
GPH = gallons per hour. Choose a filter rated at or above the upper end of the range.
Turnover rates by tank type
"Turnover" means how many times per hour the filter processes your tank's full volume. Messier fish need higher turnover:
| Tank type | Turnover | Example: 20-gal tank |
|---|---|---|
| Tropical community | 4â6Ã per hour | 80â120 GPH |
| Planted tank | 4â5Ã per hour | 80â100 GPH |
| Betta | 3â4Ã per hour | gentle, adjustable flow |
| African cichlids | 6â8Ã per hour | 120â160 GPH |
| Goldfish | 6â10Ã per hour | 120â200 GPH |
| Saltwater reef | 10â20Ã per hour | 200â400 GPH |
What GPH ratings really mean
A filter's advertised GPH is measured empty, with no media and zero head height. In real use, expect 20â30% less actual flow. That's why the calculator recommends the upper end â and why oversizing your filter slightly is almost always a good idea. You can always baffle the output; you can't make an undersized filter work harder.
Filter types at a glance
- Hang-on-back (HOB) â easy, affordable, great for tanks up to ~55 gallons.
- Canister â powerful, quiet, lots of media space; the best choice for 40+ gallons.
- Sponge â gentle, cheap, perfect for bettas, shrimp and fry tanks.
- Internal â compact option for small tanks and quarantine setups.
Frequently asked questions
What size filter for a 20 gallon tank?
For a tropical community: 80â120 GPH. For goldfish in the same tank: 120â200 GPH. Pick a filter rated at or above the top of your range.
Can a filter be too strong?
For bettas, shrimp and other slow swimmers, yes â strong currents cause stress. Use an adjustable-flow filter or baffle the output with a sponge.
Is a bigger filter always better?
Almost. More filtration means more biological media and cleaner water. The only downsides are cost and, for delicate fish, too much current.