How many fish can your tank hold?

Why one inch per gallon fails, and what this site does instead.

The old rule says one inch of fish per gallon. It works roughly for small, slim fish and fails badly for large ones. A seven-inch goldfish produces far more waste than seven one-inch tetras.

How the planner estimates it

Each species is weighted by how much waste an adult produces. A neon tetra counts as 0.8, a bronze corydoras as 1.5, an angelfish as 8 and a fancy goldfish as 15. The total is compared with the tank's volume in gallons, so 100 percent means full.

It is an estimate, not a measurement. Stay under about 85 percent while you are learning.

What moves the real limit

• Filtration. A bigger filter holds more bacteria.
• Live plants, which take up ammonia and nitrate.
• How often you change the water.
• Swimming room. An active fish in a tank that is too short is stressed even when the water is clean.