Choosing a Filter
A filter does three jobs at once, and getting its type and size right matters more than the price on the box.
Key facts
- Every aquarium filter does three jobs at once — mechanical, biological and chemical — and biological is the one you can't skip.
- Manufacturers rate flow on an empty filter; loaded with media and lifting water, real-world flow usually runs lower — size above the calculated minimum.
- Turnover-rate guidance varies by source: roughly 4x tank volume per hour for lightly stocked tanks up to 8-10x for heavily stocked or messy fish.
A filter's three jobs
Mechanical filtration traps suspended particles. Chemical filtration — activated carbon, for example — temporarily absorbs some dissolved substances and is not always used continuously. Biological filtration is literally where the nitrogen-cycle bacteria live: the filter media itself gives them a surface to colonize, which is why this job cannot be skipped to save money.
Which type fits your tank
- Sponge and internal filters — gentle flow, suited to fry, shrimp and quarantine tanks: fish can't be pulled in, and the media rinses clean without wiping out the bacteria.
- Hang-on-back (HOB) power filters — a common all-purpose choice for small-to-medium community tanks.
- Canister filters — the largest media capacity and flow, suited to larger or heavily stocked tanks.
Sizing it: turnover rate
Hobbyist guidance commonly cites roughly 4-6x tank volume per hour for lightly stocked or planted tanks, 6-8x for a typical community tank, and up to 8-10x for heavily stocked or messy fish such as goldfish and large cichlids. Sources vary on the exact figures, so treat this as a starting point rather than a strict rule.
One detail worth knowing: a manufacturer's rated flow (GPH or l/h) is usually measured on an empty filter. Loaded with media and lifting water to height, real-world flow commonly runs noticeably lower — so the standard advice is to size above your calculated minimum rather than right at it.
Gentle flow matters for some fish
Bettas, fry and shrimp generally cope worse with strong water flow — hobbyists commonly recommend a gentler option, such as a sponge filter, over a powerful HOB or canister for these.