Aquarium Fish Stocking Level Calculator

Aquarium Stocking Guide

The Stocking Rules

The traditional '1 inch of fish per gallon' rule (2.5cm per 4.5L, or roughly 1cm per 2L) is a starting point but has significant limitations. It treats a 5cm fish the same regardless of whether it is a slim tetra or a chunky goldfish. More accurate methods account for fish body shape, oxygen requirements, waste production, and territorial needs. A better rule for tropical community fish: 1cm of adult fish per litre for slim-bodied fish, 1cm per 2 litres for deep-bodied or territorial species.

Goldfish and Cold Water

Goldfish are among the heaviest waste producers in the hobby — they should never be kept in bowls or small tanks. A single fancy goldfish needs minimum 75 litres; slim-bodied varieties (comet, shubunkin) need minimum 150 litres for a single fish and an additional 50 litres per additional fish. Outdoor ponds: minimum 1,000 litres for 3–4 goldfish. Common misconception: goldfish do not grow to the size of their container — they produce hormones that stunt growth, leading to organ damage. A stunted

The Nitrogen Cycle

All aquatic life produces ammonia as waste. In a cycled aquarium, beneficial bacteria (Nitrosomonas) convert ammonia to nitrite, and Nitrobacter converts nitrite to nitrate. Nitrate is less toxic but still harmful in high concentrations — regular water changes remove it. An uncycled tank has no beneficial bacteria and ammonia builds rapidly to lethal levels. New tanks take 4–8 weeks to cycle. Cycling with fish causes significant stress and deaths. Fishless cycling (adding ammonia directly or usi

Overstocking Signs

Signs your tank is overstocked: fish gasping at the surface (oxygen depletion), frequent disease outbreaks (suppressed immune systems from stress), algae blooms (excess nutrients from waste), cloudy water (bacterial bloom from high ammonia/nitrite), and aggressive behaviour (territorial stress from crowding). Regular water testing (ammonia should be 0, nitrite 0, nitrate under 20 ppm) is the most reliable indicator of appropriate stocking density.