Aquarium Stocking Guide: How Many Fish Per Gallon?

If you're new to the hobby, you've probably heard the old "one inch of fish per gallon of water" rule. It sounds simple and easy to follow, but unfortunately, it's dangerously misleading.

Think about it this way: Does a 10-inch Neon Tetra (if one existed!) have the same impact on a tank as a 10-inch Oscar fish? Definitely not. The Oscar is a thick, muscular fish that produces a massive amount of waste, while a thin tetra produces very little.

Understand Fish Bioload and Waste Output

The most important thing isn't just the length of the fish, but their mass and waste output (bioload). Some fish are "messy" eaters and poop machines, while others are much cleaner.

• Messy Fish: Goldfish, Plecos, Cichlids. These need much more than 1 gallon per inch.
• Cleaner Fish: Tetras, Rasboras, Guppies. These have a lighter bioload.

The Surface Area Rule

Fish don't just need space to swim; they need oxygen! Oxygen enters the water at the surface. This means a shallow, wide tank can actually support more fish than a tall, narrow tank of the same volume because it has more surface area for gas exchange.

Test Water Quality to Prevent Overstocking

No calculator is perfect. The only scientifically accurate way to know if your tank is overstocked is to check your water quality.