Fish Stocking Safety Calculator.

It is a rough starting point, not a reliable formula. The rule ignores body mass, waste output, activity level, territorial behaviour, and filtration capacity. A single six inch oscar and six one inch neon tetras both total six inches, yet their bioload and space needs are completely different. Treat the result here as a sanity check, then research the specific adult size and temperament of every species you plan to keep.

For small community species of roughly one to two inches, most 20 gallon tanks comfortably support around 10 to 15 fish with good filtration and weekly water changes. That figure drops sharply for larger or messier species, and schooling fish need to be kept in groups of six or more. Always plan for adult size rather than the size at purchase.

Use net volume, the water actually in the tank after substrate, rocks, and equipment displace their share. Stocking against the label overestimates capacity by 10-15% on a typical setup. The <a href="/calculators/tank-volume">volume calculator</a> on this site reports an estimated net figure you can enter here.

Saltwater holds less dissolved oxygen than freshwater at the same temperature, marine fish tend to be more territorial, and reef invertebrates are far less tolerant of ammonia and nitrate swings. A common marine guideline is around one inch of fish per three gallons, roughly a third of the freshwater allowance, which is why the calculator applies that stricter ratio when you select a marine system.

Watch for persistent detectable ammonia or nitrite, nitrate that climbs quickly between water changes, fish gasping near the surface or at the filter outflow, increased aggression, clamped fins, and frequent disease outbreaks. Rising nitrate despite regular maintenance is usually the earliest measurable warning.

A weekly change of 20-25% suits most moderately stocked community tanks. Heavily stocked systems, or any tank where nitrate climbs above about 40 ppm between changes, benefit from larger or more frequent changes. Let test results guide the schedule rather than a fixed calendar, and always match temperature and dechlorinate replacement water.