Aquarium Stocking Calculator

Aquarium Stocking Guide

The Inch-Per-Gallon Rule (and Its Limits)

The classic '1 inch of fish per gallon' rule provides a rough starting point but has significant limitations. It does not account for fish body shape (a 6-inch Oscars needs far more space than a 6-inch neon tetra school), bioload (goldfish produce far more waste than equivalent-sized tetras), swimming patterns (active schooling fish need open swimming space), or aggression. Use it as a maximum ceiling, not a target to aim for.

Bioload and Filtration

Filtration is as important as tank volume. The nitrogen cycle (ammonia → nitrite → nitrate) depends on beneficial bacteria in your filter. A filter should turn over the full tank volume 4–10 times per hour. Heavily stocked tanks or messy fish (goldfish, cichlids, plecos) need the higher end. Never clean the filter and perform a large water change simultaneously — you risk crashing your cycle by removing too much bacteria at once.

Community Tank Compatibility

Beyond numbers, fish compatibility matters enormously. Fin-nippers (tiger barbs, serpae tetras) should not be kept with long-finned fish. Aggressive cichlids need significantly more space than peaceful community fish. Schooling fish should be kept in groups of 6+ of the same species. Bottom-dwellers (corydoras, loaches), mid-water fish (tetras, barbs), and top-swimmers (hatchetfish, danios) can share a tank without competing for the same space.

Cycling a New Tank

Never add fish to an uncycled tank. The nitrogen cycle takes 4–8 weeks to establish. Signs of completion: ammonia and nitrite both read 0, nitrates present. Fishless cycling (using ammonia directly) is the most humane approach. Once cycled, add fish gradually — adding too many at once overwhelms the biological filter. The first signs of an overstocked or uncycled tank are fish gasping at the surface and ammonia or nitrite spikes.