Aquarium stocking calculator

How the calculator works

The old "1 inch of fish per gallon" rule fails because it treats a long, thin fish the same as a heavy one, and ignores activity and behavior. Our calculator uses three pieces of data from each species profile instead:

• Adult size: fish are counted at their adult size, not the size in the store. Body mass grows much faster than length, so larger fish weigh more in the calculation.
• Minimum group: schooling and social species need a minimum number. The calculator warns you when you plan fewer.
• Minimum tank size: if a species needs more volume than your tank offers, it is flagged regardless of how full the tank is.

How to read the results

The result shows an approximate stocking level as a percentage. Up to about 80 % is a comfortable load for a well-filtered tank with regular maintenance. Between 80 and 100 % is possible with strong filtration and plants, but leaves less margin for mistakes. Above 100 % the tank is overstocked: expect water quality problems, stress and aggression.

Warnings about group size or tank size matter as much as the percentage. A tank can be at 40 % and still be wrong for a fish that needs more swimming length.

What changes the real capacity

Use the net volume of your tank (see the volume calculator). Good filtration, dense planting and frequent water changes give you some margin; a small filter, few plants or messy fish such as goldfish and large cichlids reduce it. Territorial species need floor space, not just liters.

Stock gradually after the tank is cycled, adding a few fish every one or two weeks, and check ammonia and nitrite after each addition. Then confirm the mix with the compatibility checker.