Enter your tank's volume and type to get a safe starting estimate — adjusted for filtration, not just a flat rule of thumb.
Stocking level is one input among several that keep a tank stable.
This estimate is only as accurate as the gallons you enter — confirm your tank's true water volume first.
Use the volume calculatorA correctly stocked tank still needs a filter rated to turn over its full volume several times an hour.
Review the assumptionsTwo fish can fit the stocking number and still be a bad match — temperament and adult size matter as much as count.
Browse fish guidesHeavier stocking means more frequent water changes to keep nitrates from building up between cleanings.
Review the assumptionsIt's a rough starting point, not a biological rule — real capacity depends on surface area, filtration, and species behavior far more than raw gallons. Two tanks with identical volume but different shapes can safely hold different amounts of fish. Use this calculator's result as a ceiling to stay under, not a target to hit.
Oxygen enters water through the surface, not the total volume, so a long, shallow tank supports more fish than a tall, narrow one holding the same gallons. This calculator estimates from volume alone, so treat its result as a starting point and lean conservative if your tank is tall and narrow.
Goldfish produce significantly more waste than most tropical fish and grow much larger than their pet-store size, so they're calculated separately here at roughly 20 gallons for the first fish and 10 gallons per additional fish. Fancy varieties can be kept slightly closer to this minimum than single-tailed common or comet goldfish.
Stocking level is only one factor — an immature nitrogen cycle, an undersized filter, overfeeding, or infrequent water changes will cause instability even in a lightly stocked tank. Test your ammonia, nitrite, and nitrate before assuming the fish count is the problem.
No — even a tank sized correctly for its final stocking level should be stocked gradually, a few fish every 1-2 weeks, so your filter's beneficial bacteria can grow to match the bioload. Adding everything on day one is one of the most common causes of a fatal ammonia spike.
Selecting semi-aggressive or cichlid tank type increases the space allocated per fish, but it doesn't account for specific territorial behavior between species. Aggressive fish often need dedicated territory regardless of available water volume, so treat the number here as a maximum, not a guarantee of compatibility.
Confirm your filter is rated to turn over this tank's volume at least 4-6 times per hour, since stocking and filtration capacity go hand in hand. Review the methodology and browse the fish guides.