How the stocking level is worked out
Every animal gets a number of bioload points based on its adult size and build, and the tank gets a capacity based on its real water volume and filter. The stocking level is one divided by the other.
Bioload points per fish. For fish under 3 inches (7.6 cm) we use the classic rule of one inch of fish per gallon of tank size. Above that size the rule breaks down, because a fish's body mass, and so the food it eats and the waste it makes, grows much faster than its length. So from 3 inches up, the points rise with the square of the length:
A 1.5-inch neon tetra scores about 1.2 points. A 12-inch oscar scores about 70: as much as 60 neon tetras, which matches what keepers of large cichlids find in practice.
Tank capacity. Capacity starts at 1.25 points per US gallon of real water. That equals the traditional one point per gallon of label size, because a set-up tank holds about 80% of its label volume. It is then adjusted for filtration: a filter turning the tank over fewer than 4 times an hour lowers it by 10–20%; 6 or more turnovers raise it by 10–15%. A heavily planted tank adds 10%, since plants take up ammonia and nitrate directly.
| Stocking level | Meaning |
|---|---|
| Up to 60% | Lightly stocked: very stable, forgiving of missed water changes |
| 60–95% | Well stocked: the range to aim for |
| 95–110% | Fully stocked: works with strong filtration and weekly 30–50% water changes |
| Over 110% | Overstocked: water quality will slide between changes |
What the warnings check
Volume is only half of stocking. The calculator also checks each species against the tank and against each other:
- Minimum tank size for the adult fish, however few you keep.
- Tank length: a tank should be at least four times the length of the adult fish.
- Group size: schooling and shoaling fish kept in too small a group become stressed, shy or nippy.
- Temperament: aggressive fish with peaceful ones, and fin-nippers with long-finned fish.
- Predation: fish that grow big enough to swallow their tankmates.
- Temperature and pH: whether one range suits everything on the list.
The limits of any stocking calculator
A stocking figure is a planning aid, not a guarantee. It assumes a filter that has finished the nitrogen cycle, adult fish, sensible feeding and a weekly water change. Real tanks vary: individual fish have their own temper, and a tank can be within its bioload yet too cramped for a territorial pair. Add fish a few at a time over several weeks, test the water, and let the fish's behaviour have the last word.
Common mistakes
- Stocking for the size of fish in the shop. Most are sold as juveniles: plan for the adult size.
- Adding all the fish at once. The filter bacteria need time to grow: add a few fish every one to two weeks.
- Keeping schooling fish in twos and threes. Six is the usual minimum, and more is better.
- Trusting "one inch per gallon" for big fish. A 10-inch fish is not equal to ten 1-inch fish: it produces many times the waste.
- Using the label volume. Use the real water volume from the volume calculator.
Questions and answers
How many fish can I put in my tank?
For small fish under 3 inches, about one inch of adult fish per US gallon of tank size (roughly 1 cm per 1.5 litres) is a safe guide. Larger fish need far more: use the calculator, which scales the load with body size and checks the minimum tank for each species.
Is the one inch per gallon rule accurate?
Only for small, slim fish. It badly underestimates large or heavy-bodied fish such as goldfish, oscars and plecos, and it says nothing about swimming room, aggression or group size. This calculator keeps the rule for small fish and corrects it for big ones.
How many fish can go in a 10 gallon tank?
A 10-gallon tank holds about 8 gallons of water, which suits roughly 8–10 small fish such as ember tetras or chili rasboras, or one betta with shrimp and snails. Goldfish, angelfish and most livebearers other than guppies and Endlers outgrow it.
How many fish can go in a 20 gallon tank?
About 16–18 gallons of real water gives room for around 12–16 small fish: for example 8 neon tetras, 8 pygmy corydoras and a honey gourami. See our guide for worked examples.
Does a bigger filter let me keep more fish?
A little. Stronger filtration processes waste faster and adds oxygen, but it does not add swimming room or dilute nitrate, which only water changes remove. The calculator allows up to 15% extra for strong filtration.
Is this an AqAdvisor alternative?
Yes. It answers the same question, using a method we publish in full on this page, works on phones, and shows results in both gallons and litres.
Related guides
- How to Set Up Your First Aquarium, Step by Step
- How Many Fish Can I Put in My Tank?
- The Aquarium Nitrogen Cycle, Explained