Stocking an aquarium
AquariumHQ does not publish a capacity figure, because the question 'how many will fit' is the wrong question. What matters is whether each animal in the plan can reach adult size, hold a group or a territory, and live in water the filtration can keep clean.
Short answer
Stocking is decided by adult size, footprint, social requirements, territory and filtration — not by a number of fish per unit of volume.
What actually binds
- Adult size, not purchase size. Almost everything in a shop is a juvenile.
- Footprint. Length and width decide whether a fish can swim and whether territories can coexist.
- Social minimums. One fish of a species needing twelve is a stocking error, not a small group.
- Territory. Two territorial fish in a tank with room for one territory is a fight, regardless of volume.
- Filtration and oxygen. Waste scales closer to the cube of length than to length.
On the inch-per-gallon rule
The rule persists because it is simple. It fails because waste output scales with roughly the cube of length while the rule is linear, so it treats ten 2.5 cm tetras as equivalent to one 25 cm fish. It also ignores footprint, social requirements, territory and oxygenation entirely, and it produces absurd answers outside the small-community-fish range it was invented for. AquariumHQ discusses it as an illustration and never uses it to produce advice.