Stocking Level Estimator
A starting-point estimate of how many fish fit your tank, by species adult size and the inch-per-gallon guideline.
Based on a rough 1 inch of adult fish per gallon of usable water.
This is a rough starting-point rule of thumb, not a substitute for researching each species' territory, temperament, and bioload. An angelfish is not six neon tetras: adult size alone doesn't capture how much space, filtration, or personal space a fish actually needs.
How it works
The estimator takes your tank size, knocks off 15 percent for substrate, hardscape, and equipment that displace water, then applies the old inch-per-gallon guideline using the species' typical adult length. It is a starting point for planning a shopping list, not a stocking plan you should follow blindly.
A 20-gallon tank has about 17 gallons of usable water once you account for gravel and decor. Divide that by a neon tetra's adult length of roughly 1.5 inches and you get 11 fish. Run the same 20 gallons for angelfish, which grow to about 6 inches, and the number drops to 2. The math is simple on purpose; what it leaves out is the part that actually matters.
Inch-per-gallon ignores body shape, swimming pattern, territory, and how much waste a fish produces. A neon tetra is a slim, mid-water schooler that barely disturbs the bioload. An angelfish is a tall-bodied cichlid that stakes out territory and eats far more. Two very different fish can share the same "length" number and need completely different amounts of real space.
FAQ
Is inch-per-gallon actually accurate?
It is a rough shortcut, not a rule grounded in bioload science. It works reasonably well for small, slim schooling fish and badly for large, deep-bodied, or territorial species. Treat the number this tool gives you as a ceiling to research against, not a target to hit.
Why does the tool subtract 15 percent before calculating?
Substrate, rockwork, driftwood, and equipment like filters and heaters all take up space that fish can't swim through. Subtracting a chunk up front keeps the estimate closer to the water your fish will actually have.
Can I mix species and just add up the counts?
Only loosely, and with caution. Schooling fish like neon tetras need to stay in groups of six or more to feel secure, and some combinations fight over territory regardless of how much room the math says is left. Check best beginner fish for a first freshwater aquarium before finalizing a mixed-species list.
What about fish that need more than the number suggests?
Some popular fish outgrow the math entirely. Goldfish, for one, need far more space and filtration than their length implies; see why goldfish need more room than you think. For species-specific tank size and tankmate notes, our betta fish care guide and neon tetra care guide go into the detail this calculator can't.