Aquarium Fish Stocking Calculator
Search and add fish species to your tank, set each species' count, and instantly see your stocking level, filter recommendation, and any compatibility conflicts. Works for freshwater and marine setups. Nothing uploaded.
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Learn more: fish stocking, bioload, and filter sizing
How the stocking percentage is worked out
Bioload is shorthand for how much waste a fish puts into the water, and the calculator turns it into points. Each fish costs its adult length in centimetres multiplied by a bioload score of 1, 2 or 3, and the tank supplies one point of capacity per litre.
A neon tetra is 4 cm at a bioload score of 1, so it costs 4 points. Twenty-five of them put a 100 litre tank at exactly 100 percent, and because the bar turns amber above 75 percent, 18 is the biggest school that still reads green.
Why total length is the wrong unit
The inch per gallon rule counts body length and stops there. Six neon tetras are 24 cm of fish for 24 points, while a single 20 cm discus is shorter overall but carries a bioload score of 3, so it costs 60 points.
Goldfish make the gap obvious. One adult goldfish at 30 cm and score 3 costs 90 points, so it reads 90 percent of a 100 litre tank on its own. The tool starts goldfish at their group minimum of two, and that pair reads 180 percent.
The filter figure comes from the tank, not the fish
The minimum filter is ten times tank volume per hour, and nothing else feeds into it. A 100 litre tank shows 1000 L/hr, and gallon mode keeps the same ratio, so a 50 US gallon tank shows 500 GPH.
Adding goldfish, plecos or an oscar does not move that number. Treat 10x as a floor and buy more flow than the tool prints once the gauge sits in the amber band.
What the warnings actually check
Four situations raise a compatibility warning: an aggressive species (betta or oscar) sharing with any peaceful one, a betta count above one, freshwater and marine species on the same list, and goldfish kept with tropical freshwater fish, since goldfish want cooler water than most tropicals at 18-22 C.
White cloud minnows are exempt from that goldfish rule because they take the same cool water. Schooling is a separate check, so a count below the species minimum (6 for neon tetras, 4 for corydoras) warns you even while the gauge is still green.
FAQ
How many neon tetras fit in a 100 litre tank?
Twenty-five, which lands on exactly 100 percent. Each neon costs 4 points, its 4 cm adult length times a bioload score of 1, against 100 points of capacity. The bar turns amber above 75 percent, so 18 is the largest school that still reads green.
Does one goldfish overstock a 100 litre tank?
Not on its own. A 30 cm goldfish with a bioload score of 3 costs 90 points, which reads 90 percent. The catch is the group minimum of two, and that pair costs 180 points in the same tank.
Does the filter recommendation change when I add fish?
No. It is ten times the tank volume per hour whatever the fish list holds, so 100 litres always shows 1000 L/hr. Raise it yourself for a heavily stocked tank or for species that produce more waste.