How many fish per gallon?
The inch rule's failures, this calculator's point classes, and a 10-gallon walkthrough.
Where 1-inch-per-gallon breaks
Direction one: a 10-inch oscar "fits" a 10-gallon by the inch rule and has no room to turn. Direction two: twelve small fish in 10 gallons are 12 points. Capacity on a normal setup is 10, so this calculator calls that 120% and overstocked. Length is not the load.
Bioload points — the working system
| Class | Adult size | Points | Examples |
|---|---|---|---|
| Small | ≤2" | 1 | Neon tetra, guppy, cherry barb, ember tetra |
| Medium | 2–5" | 3 | Corydoras, angelfish, dwarf gourami, honey gourami |
| Large | 5–10" | 8 | Fancy goldfish, keyhole cichlid, bristlenose pleco |
| XL | 10"+ | 14 | Oscar, common pleco, silver dollar |
On a normal setup, capacity is 1 point per gallon. A heavy-stock setup uses 0.6 points per gallon. Comfortable means the result is at or under 80%.
Calculator walkthrough for a 10-gallon tank
Section added October 6, 2026. This block is the stocking function, inputs shown. Enter them on the stocking calculator. Points are a planning estimate for a cycled freshwater tank, not a published standard.
Capacity is round(10 × 1.0) = 10. Eight fish are 8 × 1 = 8 points, and round(8 / 10 × 100) = 80. Comfortable means the percent is at or under 80, so 80 is the last comfortable step on this function. Ten fish are round(10 / 10 × 100) = 100, which is at capacity (above 80 and at or under 100). Eleven fish are round(11 / 10 × 100) = 110, overstocked. The same three sizes, each run on its own, are on how many fish in a 10, 29, or 55 gallon tank. A 20-gallon walkthrough is already on how many fish in a 20 gallon tank.
FAQ
What does 8 small fish score in a 10-gallon tank?
8 points against a capacity of 10, which is 80%, and the status is comfortable because 80 is the top of that band. Eleven small fish are 11 points, 110%, and overstocked. Ten small fish are 10 points, 100%, at capacity.
Is that score a published stocking standard?
No. Points, watts, and turnover on this site are planning estimates for a cycled freshwater tank. The 10-gallon arithmetic is the calculator function with the inputs shown. It is not a university headcount.
Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.
Sources
The walkthrough uses this site's stocking function only. No outside headcount is added in this section.
The three rules that actually keep fish alive
1. Cycle first. No fish until ammonia and nitrite are gone. The clocks are on how to fix cloudy water in a new tank, and they are not one number. 2. Adult size. Enter the grown fish, not the shelf fish. 3. Add slowly. Add a small group after the cycle, not the full list on day one.
Sourced versions of those steps, each with a calculator example: how to keep pet fish alive, how to add fish, how much to feed tropical fish, and how to treat sick fish. A 500-gallon pond is not this page: how many koi in a 500-gallon pond. If a fish is past recovery, follow how to humanely euthanize a fish.
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Test kits and filters: liquid test kits on Amazon · HOB filters on Amazon
Sources & Standards
Bioload points, watts per gallon, and filter turnover on these pages are planning estimates for a cycled freshwater tank. They are not a published standard.