Aquarium Stocking Calculator

How many fish can I keep?

A common guideline is 1 cm of adult fish per 1 litre of water, or about 1 inch of fish per US gallon. Divide the water volume by the adult length of your fish to estimate the number you can keep. Treat it as a starting point: filtration, species, adult size and territory all matter, and messy or active fish need more space.

How to use the fish stocking calculator

1. Enter the water volume in litres. Use the real water in the tank, not the size on the box. If you only know the gallons, multiply US gallons by 3.785 to get litres. The water volume calculator works out the real figure from your measurements.
2. Enter the average adult length of your fish in centimetres. Use the size the fish will reach as adults, not the size in the shop.
3. Read the recommended number of fish. The calculator divides the litres by the fish length, using the guideline of 1 cm of adult fish per litre of water.

Example: a tank with 64 litres of water and fish that grow to 3 cm gives 64 / 3 = 21 fish.

How many fish for common tank sizes

The table below uses the real water volume (about 85 percent of the tank size, to allow for gravel, decor and the gap at the top) and the same 1 cm per litre guideline as the calculator.

Treat these as the upper limit, not a target. A new tank should be stocked slowly, a few fish at a time.

Planning a mix of species

Most tanks hold more than one kind of fish. To check a mixed list, add up the adult length of every fish you plan to keep and compare the total with the litres of water:

• 8 fish at 3 cm = 24 cm
• 6 fish at 2.5 cm = 15 cm
• 1 fish at 5 cm = 5 cm
• Total: 44 cm, which fits within a tank holding 64 litres of water with room to spare.

When the guideline does not work

The 1 cm per litre rule was made for small, slim fish. Stock well below it when:

• The fish are heavy-bodied or messy, such as goldfish, large cichlids and plecos. They produce far more waste per centimetre than a tetra.
• The fish are large. A single 30 cm fish does not fit in a 30 litre tank, even though the arithmetic says it does. Big fish need room to turn and swim.
• The fish are territorial. Some species need their own area of the tank, regardless of the numbers.
• The filter is small or water changes are irregular.

Before you add fish

Make sure the tank is cycled, so the filter bacteria can handle the waste. See how to cycle a fish tank. For example stocking lists by tank size, read how many fish in a 10 gallon tank, and for bettas, see what size tank a betta fish needs.