🐠 Aquarium Fish Stocking Calculator
Estimate how many fish your aquarium can safely support using the surface-area stocking rule, which accounts for oxygen exchange at the water’s surface.
What this calculator does
Aquarium stocking is often estimated with the simple ‘inch per gallon’ rule, but that ignores tank shape and how oxygen actually reaches the water: oxygen exchange happens at the water’s surface, not through the whole volume. The surface-area rule accounts for this by basing capacity on the tank’s water-surface footprint.
How the math works
The calculator multiplies tank length by width to get surface area in square inches. It then divides by a guideline of square inches needed per inch of fish: roughly 12 square inches per inch of fish for typical tropical fish, and roughly 30 square inches per inch for goldfish and other coldwater fish, which need more dissolved oxygen. Comparing your total fish length (the sum of the adult length of every fish) against that capacity shows how stocked the tank is.
Stocking guidelines vary by species, filtration, water changes, and plant life, so use this as a general starting guideline rather than an absolute limit, and research the specific needs of your fish species.