Aquarium stocking calculator
Add the fish you keep or plan to keep. The calculator estimates bioload against your filtration and flags tank size, group size and temperature problems.
How stocking is estimated
The old "one inch of fish per gallon" rule treats a 6-inch fish like six 1-inch fish. In reality waste output follows body mass, which grows much faster than length: a 4-inch fish produces roughly four times the waste of a 2-inch fish, not twice. This calculator scores each fish as (adult length in inches)² ÷ 2, adjusted for species that are unusually messy (goldfish, plecos, oscars) or light (shrimp and snails). A tank with a standard filter handles about 2 points per gallon of real water; strong filtration (8× turnover or more) adds 20%, and dense live plants add 10%.
What the levels mean
- Under 50%: lightly stocked. Plenty of margin for growth and mistakes.
- 50–85%: a comfortable, stable community.
- 85–100%: fully stocked. Weekly water changes are not optional.
- Over 100%: overstocked. Expect high nitrate and stress unless you add filtration and change more water.
Beyond bioload
A tank can be lightly stocked and still wrong. The calculator also checks each species' minimum tank size, the group size schooling fish need to feel secure, whether everyone shares a temperature range, and known conflicts such as fin-nippers with long-finned fish or angelfish with fish small enough to eat. It uses adult sizes: buy for the fish they will become.
This is an estimate to support research, not a guarantee. Water hardness, aquascape, individual temperament and maintenance all matter.
FAQ
How many fish can I put in a 20-gallon tank?
About 15–25 small fish such as tetras, rasboras and pygmy corydoras. For example, 10 neon tetras and 6 pygmy corydoras come to about 35% on this calculator, leaving room for a centerpiece such as a honey gourami.
Is the one inch per gallon rule accurate?
Only for small, slim fish. It underestimates the waste of large or deep-bodied fish and ignores territory, swimming space and group size.
How many neon tetras in a 10-gallon tank?
Six to eight neon tetras on their own; a group of 6 with a few shrimp or a nerite snail is comfortable. A 20-gallon long gives them more swimming room.