How Many Fish Can I Put in My Tank?

The short answer

For small community fish (under 3 inches or 7.5 cm as adults), a safe starting point is one inch of adult fish for each US gallon of tank size, or about 1 cm of fish per 1.5 litres.

Notice the second column. A tank never holds its label volume: glass, the gap at the top, gravel and decor take 15–20% away. The traditional rule already allows for that, and so does our calculator, as long as you give it the real water volume from the volume calculator.

Why "one inch per gallon" is only the start

The rule works for small, slim fish and fails badly for everything else, for three reasons.

Big fish are far heavier than their length suggests. A fish twice as long has roughly eight times the body mass, eats accordingly, and makes that much more waste. One 10-inch oscar is not "10 inches of fish": it loads a tank like 50 or 60 small tetras.

Fish need room, not just clean water. A common pleco might "fit" a 55-gallon tank by the numbers when young, but it grows to 15 inches and cannot turn around in a tank 13 inches wide. A tank should be at least four times as long as the adult fish.

Behaviour changes the count. Schooling fish need groups of six or more to feel safe. Territorial fish need their own space however clean the water is. Some fish simply eat others.

Our stocking calculator handles all three: it scales each fish's load with its adult body size, checks the minimum tank for each species, and warns about group size, aggression and mismatched water.

Example stocking lists

Each list below was run through the stocking calculator at adult sizes, with a filter turning the tank over five times an hour. Treat them as starting points you can open and edit.

10 gallon tank (38 litres)

Nano community: 8 ember tetras, 10 cherry shrimp, 1 nerite snail. Stocking level 70%. A bright school with a clean-up crew. Plant it densely so the shrimp can breed. Open this list in the calculator

Betta tank: 1 betta, 6 cherry shrimp, 1 nerite snail. Stocking level 40%. Lightly stocked on purpose: a betta wants calm water and room of its own. Some bettas hunt shrimp, so give them moss to hide in. Open this list in the calculator

20 gallon tank (76 litres)

Classic small community: 8 neon tetras, 8 pygmy corydoras, 1 honey gourami. Stocking level 81%. Fish for every level: the tetras in the middle, the gourami at the top, the pygmy corydoras on the bottom. Open this list in the calculator

20 long, two schools: 7 harlequin rasboras, 6 panda corydoras. Stocking level 92%. A 20 long has the floor area panda corydoras need. Use sand to protect their barbels. Open this list in the calculator

29 gallon tank (110 litres)

Peaceful community: 8 harlequin rasboras, 6 panda corydoras, 1 honey gourami. Stocking level 68%. Leaves margin to add a second small school later, such as six cherry barbs. Open this list in the calculator

Barbs and corydoras: 8 cherry barbs, 6 bronze corydoras. Stocking level 83%. Cherry barbs are the peaceful barb. A hardy pair of species for a first tank. Open this list in the calculator

55 gallon tank (208 litres)

Angelfish community: 2 angelfish, 10 rummy-nose tetras, 6 Sterba's corydoras. Stocking level 86%. Rummy-nose tetras are big enough to be safe with adult angels, and Sterba's corydoras enjoy the same warm water. Open this list in the calculator

Rainbow and rasbora tank: 8 dwarf neon rainbowfish, 8 harlequin rasboras, 6 kuhli loaches. Stocking level 87%. An active, colourful tank that uses the full four feet of swimming length. Open this list in the calculator

75 gallon tank (284 litres)

One oscar: 1 oscar. Stocking level 81%. One adult oscar fills a 75-gallon tank by itself. Use heavy filtration and change half the water every week. Open this list in the calculator

Large rainbow community: 6 Boeseman's rainbowfish, 8 bronze corydoras, 1 bristlenose pleco. Stocking level 82%. Boeseman's rainbowfish need the length and show their best colour in a group. Open this list in the calculator

Fancy goldfish: 3 fancy goldfish. Stocking level 60%. Three fancy goldfish with room to grow. No heater is needed in a normal room, but filtration must be strong. Open this list in the calculator

Fish that need more room than people expect

Shops sell most of these at an inch or two long. Stock for the adult.

How to stock a new tank

1. Finish the nitrogen cycle first. An uncycled filter cannot process any fish load.
2. Add fish in stages. A third of the list at a time, a week or two apart, testing for ammonia and nitrite between.
3. Stop at about 85–90%. The spare capacity is your margin for a missed water change, a hot week or a fish that grows bigger than the book says.
4. Watch the fish. Gasping at the surface, clamped fins, hiding and chasing all tell you more than any calculator.

Questions and answers