Aquarium stocking calculator
Bioload beats the inch-per-gallon myth. Count your fish by adult size and waste class — get stocking points, heater watts, filter GPH, and your water-change schedule.
Test kits, filters, and tank deals linked after affiliate approval.
Why bioload beats inches
The 1-inch-per-gallon rule dates from goldfish-bowl era advice. It ignores that a 10-inch oscar weighs 40× a 10-inch school of neons, that surface area limits oxygen exchange more than volume limits waste, and that adult size is what counts — the 2-inch bala shark you bought today wants 150 gallons in three years. Bioload points approximate real waste output: small (≤2") = 1 pt, medium (2–5") = 3 pts, large (5–10") = 8 pts, XL (10"+) = 14 pts, with messy fish (goldfish, common plecos) counted one class up.
Quick reference by tank size
| Tank | Capacity (pts) | Heater (78°F from 68°F) | Filter GPH (5×) |
|---|---|---|---|
| 10 gal | 10 | 50 W | 50–80 |
| 20 gal long | 20 | 75–100 W | 100–150 |
| 29 gal | 29 | 100–150 W | 150–200 |
| 55 gal | 55 | 200–250 W | 275–350 |
| 75 gal | 75 | 250–300 W | 375–500 |
Capacity assumes normal filtration, regular water changes, and a cycled tank. Planted tanks with fast growers buy you maybe 10–20% headroom — don't spend it all.
Guides
How many fish per gallon? The honest answer
Why the inch rule fails, how bioload points work, and worked examples for 10, 20, and 55 gallon tanks.
Aquarium heater wattage: the 3–5 W/gal rule, done right
ΔT math, two-heater redundancy for big tanks, and why an oversized heater with a thermostat beats undersized.
Power outages and your tank ↗
A stocked tank on a battery backup — GeneratorSizer and Battery Bank Sizer do the runtime math.