It uses the classic 'one inch of adult fish per gallon' guideline, adjusted for filtration. It's a starting point, not gospel — body mass matters more than length (a 6-inch goldfish produces far more waste than six 1-inch tetras), and schooling species need groups and swimming room.
Stock responsibly
Always check adult size, temperament, and schooling needs of each species, and cycle your tank before adding fish. When in doubt, under-stock — it's far more forgiving for water quality and fish health.
FAQ
How many fish can I put in a 10 gallon tank?
Roughly 8–10 inches of small adult fish total — for example a school of 6–8 small tetras or a betta with a few corydoras. Avoid large or messy fish in a 10 gallon.
Is the 1 inch per gallon rule accurate?
It's a rough starting guide for small community fish. It breaks down for large, messy, or territorial species — use it to ballpark, then research each fish's real needs.