Aqueon’s filter guide says a filter should move four to six times the tank’s volume of water an hour, so a 20 gallon tank takes 80 to 120 gallons an hour.
Filter flow rate
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Where the number comes from
The four to six times figure is from Aqueon, a filter maker, in an article on choosing a filter, and it says a hardworking filter moves at least four times the tank’s volume an hour. RSPCA and OATA say every fish tank needs a well set up filter, but neither gives a flow rate.
Marineland’s Penguin Pro 125 is rated higher: it moves 125 gallons an hour and is rated for aquariums up to 20 gallons, about 6.3 times a 20 gallon tank’s volume an hour.
Gentle flow for some fish
Some fish want less current. Aqueon names bettas among the fish with delicate fins that need a gentler flow. RSPCA Australia says bettas prefer low flow and asks for a filter with adjustable flow, and OATA says fancy goldfish prefer a gentle current.
Common questions
What does GPH mean on a filter?
Gallons per hour: the water the filter moves in an hour. Marineland's Penguin Pro 175 is rated 175 GPH for aquariums up to 30 gallons.
How do I clean a filter without losing its bacteria?
OATA says never to rinse filter media under a tap, because that washes away and kills the beneficial bacteria.
The flow rate comes from Aqueon’s filter guide, filter ratings from Marineland, and the care notes from RSPCA, RSPCA Australia, OATA and a University of Illinois veterinary column. Checked against published data from OATA, NIST, RSPCA, RSPCA Australia and University of Illinois (plus 2 more).
RSPCA, Ornamental fish: Environment (“Choosing an aquarium”)
OATA (Ornamental Aquatic Trade Association), How to care for bettas, September 2022 (PDF)
OATA, How to care for fancy goldfish, January 2023 (PDF)
OATA, How to set up and maintain a freshwater aquarium, September 2022 (PDF)
RSPCA Australia knowledgebase, How should I care for my Siamese fighting fish?
University of Illinois College of Veterinary Medicine, “Bettas Need More Than Bowls” (pet health column, 2019)
Aqueon, “How to Choose the Best Filter for You and Your Tank” (maker article)
Marineland, Penguin BIO-Wheel Power Filters (product page)
General care guidance from animal welfare charities, vets and makers' published sizes. It isn't veterinary advice: if your pet seems unwell or has eaten something it shouldn't, call your vet.