Tetra’s general rule is one inch of adult fish per gallon, RSPCA’s is 1.5 to 2 liters of water for each centimeter of tropical fish, and OATA says no one can give an exact number.
Fish
Without the tail, for RSPCA’s rule.
What each source allows
Why the rules give different numbers
One inch of fish per gallon works out to about 1.49 liters per centimeter, the most generous end of RSPCA’s range for tropical fish. RSPCA’s goldfish rule, 2 liters per 0.5 cm, is 4 liters per centimeter, about 2.7 times as much water.
Most species rules don’t count length. OATA gives goldfish 100 liters for the first adult and 50 for each after, and RSPCA 60 liters each. RSPCA Australia is the exception: it sizes a goldfish tank from the fish’s adult length. A male betta never shares a tank with another male, whatever its size.
What else limits the number
Tetra lists what its rule leaves out: how big the fish will be as an adult, whether it’s active, whether it schools in groups of five or more, how much waste it makes and how it gets on with others. RSPCA says water quality also depends on temperature, the water source, filtration and how well the tank is kept.
OATA warns not to expect to stock a tank as heavily as a shop does: shops manage their tanks with more expertise and stronger filtration. Stocking too many fish, or too fast, can cause new tank syndrome, so add fish a few at a time. See how to cycle a new fish tank.
Common questions
How many fish can go in a 10 gallon tank?
Tetra's inch per gallon rule allows 10 inches of adult fish, and RSPCA's 1.5 to 2 liters per centimeter allows about 19 to 25 cm (7.4 to 9.9 in) of fish, tails not counted. RSPCA also says to choose a tank bigger than about 40 liters, and a 10 gallon holds about 37.9.
How many fish can go in a 20 gallon tank?
Tetra's general rule allows 20 inches of adult fish in a 20 gallon tank, which holds about 75.7 liters, and RSPCA's tropical fish rule about 37.9 to 50.5 cm (14.9 to 19.9 in), tails not counted.
Does the inch per gallon rule work for goldfish?
RSPCA gives goldfish 4 liters of water per centimeter of length, about 2.7 times what Tetra's inch per gallon allows, and OATA gives one adult goldfish at least 100 liters.
The stocking rules come from Tetra, RSPCA, RSPCA Australia, OATA and a University of Illinois veterinary column. Checked against published data from OATA, RSPCA Australia, NIST, Tetra and RSPCA (plus 1 more).
Tetra, Complete LED Aquarium Kit, 10 gallon (product page)
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 guppies and mollies, September 2022 (PDF)
OATA, How to care for American cichlids (includes angelfish), September 2022 (PDF)
OATA, How to care for fancy goldfish, January 2023 (PDF)
OATA, How to care for goldfish in aquaria, January 2023 (PDF)
OATA, How to care for freshwater shrimp and snails, 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?
RSPCA Australia knowledgebase, How should I care for my goldfish?
University of Illinois College of Veterinary Medicine, “Bettas Need More Than Bowls” (pet health column, 2019)
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.