Aquarium Stocking Calculator

How Many Fish Can My Tank Hold?

The old “one inch of fish per gallon” rule is dangerously oversimplified. A 10-inch Oscar produces far more waste than ten 1-inch neon tetras. This calculator uses a bioload system that accounts for fish size, waste production, and water type for a much more accurate result.

Freshwater vs Saltwater Stocking

Saltwater tanks can support roughly half the bioload of a freshwater tank of the same size. Marine fish are more sensitive to water quality, ammonia spikes are more dangerous in saltwater, and the biological filtration in reef tanks is more complex to establish.

The Nitrogen Cycle — Most Important Thing to Understand

Never add fish to an uncycled tank. The nitrogen cycle establishes the beneficial bacteria that convert toxic ammonia (from fish waste) into nitrite and then into relatively harmless nitrate. Cycling takes 4–6 weeks. Adding fish too early causes “new tank syndrome” and kills fish.

Schooling Fish Rules

Many popular fish are schooling species that experience stress, lose colour, and become aggressive when kept in groups that are too small. Neon tetras, tiger barbs, corydoras, and most danios need a minimum of 6 fish. The calculator will warn you if your group is too small.

Filtration Matters as Much as Tank Size

A well-filtered 20 gallon tank can support more fish than a poorly filtered 40 gallon. As a rule, your filter should turn over the total tank volume at least 4–6 times per hour. For messy fish like goldfish and oscars, aim for 8–10x turnover.