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Fish stocking calculator
Enter your tank and your animals to see how full the tank is and whether the groups are large enough.
How it is calculated
What matters is not how many litres a tank holds but how much floor area the fish get: territories, swimming space and hiding places are spread over the area, not the height. That is why the old “one inch of fish per gallon” rule is considered outdated. A tall, narrow tank has many litres but little room.
Each fish needs floor area per centimetre of adult size: 15 cm² for species up to 5 cm, 25 cm² up to 15 cm and 40 cm² for larger ones. Large fish therefore count disproportionately. Shrimp and snails hardly load the tank and are not counted.
Example: an 80 × 35 cm tank has 2800 cm². 12 neon tetras (4 cm) need 12 × 4 × 15 = 720 cm², 6 corydoras (6 cm) 6 × 6 × 25 = 900 cm². Together that is 1620 cm², or 58 %.
What the calculator does not check
Floor area is only part of it. Whether species go together also depends on water parameters, temperature, temperament and predators: an adult angelfish eats neon tetras, however much room there is. Also watch the minimum tank size of each species: some need a longer tank, no matter how empty it is.
The app runs exactly this check: water parameters against your latest reading, minimum tank, group size, predators and territorial behaviour.
In the app: stocking with a compatibility check
In Aqualizer the stocking advisor checks every species against your tank and the animals already living in it, and suggests species that fit. The check and the top three suggestions are free.
- Compatibility and predators
- Water parameters against your reading
- “What else fits?” for your tank
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Stocking FAQ
How many fish fit in my aquarium?
That depends on the floor area and the adult size of the species. Per centimetre of fish, allow 15 cm² of floor for small, 25 cm² for medium and 40 cm² for large species. An 80 × 35 cm tank can hold, for example, 12 neon tetras and 6 corydoras with room to spare.
Does the one inch per gallon rule still apply?
No. It ignores the shape of the tank and the fact that a 10 cm fish needs far more room than ten 1 cm fish. The floor area method is more accurate and is what is recommended today.
How small can a tank for fish be?
For keeping fish permanently, 54 litres and 60 cm length are considered the minimum (report of the German Federal Ministry of Agriculture). Smaller tanks suit shrimp and snails.
Why does group size matter?
Schooling fish such as tetras or corydoras only feel safe in a group. Too few are stressed, hide or fall ill. The calculator warns when a species is below its minimum group.
Do shrimp and snails count?
Hardly. They produce little waste and use the space differently from fish, so the calculator leaves them out of the floor area.