A male betta needs a heated, filtered tank of at least 20 liters (5.3 gallons), with no other male betta in it.
Your tank
Your tank against each minimum
What to get
What to get
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Why not a bowl
A University of Illinois vet says bettas should not live in bowls, and should ideally have a 5 gallon tank or larger. RSPCA Australia says 20 liters or more lets a betta show its normal activity.
A bigger tank also holds its temperature and water quality steadier, and a wide one has more surface where oxygen gets into the water (RSPCA). See do bettas need a filter and heater for the two pieces of kit every source calls for.
Males, females and tank mates
Males are highly aggressive to each other. OATA says two males will fight until one is badly hurt or killed. Females are much calmer, and OATA keeps them in groups of at least four, in at least 60 liters. The Illinois vet says a betta shouldn’t be housed with any other betta at all.
With other kinds of fish, a male is more often the victim. OATA says males are often the target of intense fin nipping, so keep a male alone or with very placid tank mates. See betta tank mates.
Common questions
Can a betta live in a 2.5 gallon tank?
No, by any of the three sources. A 2.5 gallon tank holds about 9.5 liters, under RSPCA Australia's absolute minimum of 10 liters and half of OATA's 20 liters.
How often should I change a betta's water?
RSPCA advises a 30% water change every week for a fish tank, and OATA's setup guide says partial changes of up to 25% every week.
How big do bettas get?
OATA says bettas grow to about 7 cm, a little under 3 inches.
Care and tank sizes come from OATA, RSPCA, RSPCA Australia and a University of Illinois veterinary column, and filter flow from Aqueon. Checked against published data from OATA, NIST, RSPCA, RSPCA Australia and University of Illinois (plus 1 more).
RSPCA, Ornamental fish: Environment (“Choosing an aquarium”)
OATA (Ornamental Aquatic Trade Association), How to care for bettas, September 2022 (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)
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.