How it works
stocking % = total adult fish length ÷ (gallons × adjusted inches per gallon) × 100
The calculator starts from the long-standing community-tank guideline of one inch of adult fish per US gallon of water, then adjusts it. Filtration matters most: the rule assumes roughly four tank turnovers per hour, so a stronger filter raises the allowance and a weak one lowers it, within a sober 0.7 to 1.4 band. A densely planted tank earns a further fifteen percent because live plants consume ammonia and nitrate and add oxygen. The adjusted figure is capped at two inches of fish per gallon, the upper limit university extension guidance gives even for a well-filtered tank. Multiplying the adjusted inches-per-gallon by your gallons gives the guideline capacity in inches of adult fish; your total adult length divided by that capacity, times one hundred, is the stocking percentage. Under about eighty-five percent is understocked with room to grow, up to roughly one hundred and ten percent is well stocked, and beyond that the tank is over the guideline. Two cautions are built into the wording of the result. First, always use adult sizes: a bag of inch-long juveniles that mature at four inches will quadruple your load. Second, the inch-per-gallon rule sizes waste and oxygen demand only. It knows nothing about aggression, territory, schooling needs, swimming room or adult body mass, so a single large or aggressive fish can wreck a tank that the percentage calls comfortable. Treat the number as a starting sanity check, then research every species.
Sources
- Purdue Extension — 4-H Aquatic Science (4-H-447-W) A pair of fish totalling one inch needs a gallon of water — no more than two inches of fish per gallon with a pump and filter.
- USGS Water Science School — Dissolved Oxygen and Water Oxygen enters water mainly from the atmosphere, dissolving at the air-water surface; dissolved oxygen is essential for aquatic life, and warmer water holds less of it — the physics behind the surface-area limit on stocking.
FAQ
How many fish can I put in my tank?
As a guideline, allow about one inch of adult fish per gallon of water in a filtered community tank, and never exceed two inches per gallon. A 20-gallon tank therefore supports roughly 20 inches of small fish — say a dozen tetras plus a few corydoras. Enter your volume, filtration and stock above for an adjusted figure, then check each species has room to behave normally.
Is the one-inch-per-gallon rule accurate?
It is a useful starting point for small, slender community fish, not a law. It sizes the waste load and oxygen demand but ignores body mass, aggression and territory. A 10-inch oscar needs far more than 10 gallons, while ten 1-inch neon tetras are fine in 10 to 15 gallons. Use the rule to avoid gross overstocking, then research adult size and temperament.
Why does adult size matter more than current size?
Fish are usually sold as juveniles. A common pleco is three inches in the shop and grows past eighteen. If you stock to the size in the bag, the tank becomes badly overstocked within a year. Always total the expected adult length of every fish, which is what this calculator asks for, so today numbers reflect tomorrow bioload.
How does filtration change stocking?
Filtration removes fish waste, so more of it lets the tank carry more fish. The guideline assumes about four tank turnovers per hour. A filter rated well above that raises the allowance modestly, while an underpowered filter lowers it. Filtration cannot buy unlimited stock, though; surface area for gas exchange and the fish behavioural needs still cap the tank.
Do live plants let me keep more fish?
A densely planted tank does help. Plants take up ammonia and nitrate directly and release oxygen during the day, easing the biological load, which is why the calculator adds about fifteen percent for a planted tank. The effect is real but limited, and heavily planted tanks still need water changes and sensible stocking. It is a bonus, not a licence to overstock.
What does the stocking percentage not account for?
Behaviour, mostly. The percentage says nothing about aggression, territoriality, schooling requirements, fin-nipping, predator-prey pairings or the swimming room active fish demand. A tank at sixty percent by the numbers can still fail if an aggressive fish claims it. Always pair this guideline with species research on temperament, minimum tank footprint and compatible tank mates.
This is a guideline based on volume, filtration and total fish length — not a species database. It does not model aggression, territory, schooling needs or adult body mass. Always research each species before stocking. General guidance, not veterinary advice.