How Many Fish in a 10 Gallon Tank? (Plus 5, 20, 29 and 55 Gallon)

“How many fish can I put in my tank?” is the most common question new fishkeepers ask, and the one most often answered badly. Overstocking is behind a large share of early fish losses: too many fish produce more waste than the filter bacteria can handle, and the water quality crashes.

This guide explains the simple rule most people use, where that rule goes wrong, and gives realistic example stocking lists for the most popular tank sizes.

The quick answer: the inch per gallon rule

The classic guideline is 1 inch of adult fish per US gallon of water (roughly 1 cm per litre). Add up the full grown length of every fish you plan to keep, and keep the total at or below the number of gallons your tank holds.

• 5 gallon: about 5 inches of fish in total
• 10 gallon: about 10 inches
• 20 gallon: about 20 inches
• 29 gallon: about 29 inches
• 55 gallon: about 55 inches

The aquarium stocking calculator does this for you: enter your water volume and the average adult size of your fish, and it returns a recommended number.

Where the rule goes wrong

The inch per gallon rule is a starting point, not a guarantee. Treat it as an upper limit and keep these exceptions in mind:

• Use the real water volume. Gravel, rock and the gap at the top mean a 10 gallon tank usually holds closer to 8 or 9 gallons of water. Use the water volume calculator to get the real figure.
• Use adult size, not shop size. Fish are sold young. A 1 inch bristlenose pleco grows to around 5 inches.
• Body shape matters. The rule works for slim fish such as tetras and rasboras. Deep-bodied or heavy fish such as goldfish, cichlids and large plecos produce far more waste per inch.
• Schooling fish need groups. Tetras, rasboras and corydoras should be kept in groups of at least six, which uses up the budget quickly.
• Swimming space and territory. Active swimmers need length, and territorial fish need room to stake out a space, regardless of the inch count.
• Filtration and maintenance. A strong filter and weekly water changes give you some margin. Light filtration means you should stay well under the limit.

How many fish in a 5 gallon tank?

A 5 gallon tank has room for about 5 inches of fish, but it is too small for an active school. The best options are:

• One betta fish (about 2.5 to 3 inches), on its own or with a few snails or shrimp
• A colony of shrimp, such as cherry shrimp, with snails
• A very small group of nano fish, but only if the tank is heated, filtered and well established

Avoid goldfish, guppy colonies and schooling tetras in 5 gallons. They need more space to swim and to dilute their waste.

How many fish in a 10 gallon tank?

A 10 gallon tank (20 x 10 x 12 inches) can take about 10 inches of small fish. Example stocking lists that stay within the rule:

• Nano community: 8 ember tetras (about 0.8 in each, 6.4 in) plus 3 pygmy corydoras (about 1 in each, 3 in) = about 9.4 inches
• Betta tank: 1 betta (about 3 in) plus 4 pygmy corydoras (4 in) = 7 inches, with room for snails
• Livebearers: 3 male guppies (about 1.5 in each) plus 3 pygmy corydoras = 7.5 inches. Keep only males or you will soon have far more than three

A 10 gallon tank is not suitable for goldfish, angelfish, gouramis larger than the honey gourami, or any fish that grows beyond about 3 inches.

How many fish in a 20 gallon tank?

A 20 gallon tank can hold about 20 inches of fish. If you can choose, a 20 gallon long (30 x 12 x 12 inches) is better for active swimmers and bottom dwellers than a 20 gallon high (24 x 12 x 16 inches), because it has a larger footprint. Example lists:

• Classic community: 8 neon tetras (about 1.5 in each, 12 in) + 5 pygmy corydoras (5 in) + 1 honey gourami (about 2 in) = 19 inches
• Rasbora tank: 10 harlequin rasboras (about 2 in each) = 20 inches
• Shrimp and nano fish: a group of 12 chili rasboras (about 0.7 in each, 8.4 in) with a shrimp colony, leaving plenty of margin

How many fish in a 29 gallon tank?

A 29 gallon tank (30 x 12 x 18 inches) shares the footprint of a 20 long but is taller, giving you about 29 inches of fish. Example:

• 10 harlequin rasboras (20 in) + 1 dwarf gourami (about 3 in) + 5 pygmy corydoras (5 in) = 28 inches

Because the footprint is the same as a 20 long, do not overload the bottom with too many corydoras or loaches; they live on the floor space, not the water column.

How many fish in a 55 gallon tank?

A 55 gallon tank (48 x 13 x 21 inches) offers about 55 inches of fish and a 4 foot swimming length, which opens up larger schools and centrepiece fish. Example:

• 12 rummy nose tetras (about 2 in each, 24 in) + 8 bronze corydoras (about 2.5 in each, 20 in) + 1 pearl gourami (about 4.5 in) + 1 bristlenose pleco (about 5 in) = 53.5 inches

A 55 gallon tank is only 13 inches front to back, so it suits schooling fish and community setups better than large, territorial cichlids, which appreciate the 18 inch depth of a 75 gallon tank.

Stock slowly

Even a correctly sized stocking list should be added gradually. Start with a fully cycled tank, add a few fish at a time, and wait a week or two between additions so the filter bacteria can grow to match the extra waste. Test ammonia and nitrite after each addition.

Frequently asked questions

How many neon tetras can I keep in a 10 gallon tank? By the inch per gallon rule, about 6 neon tetras (1.5 in each, 9 in total). Neon tetras are active schoolers, so a 20 gallon long gives a group of 8 to 10 a much better home.

Can I put a goldfish in a 10 gallon tank? No. Goldfish grow large and produce a lot of waste. Even a single fancy goldfish needs far more water than 10 gallons.

Is it better to use litres? The metric version of the rule is roughly 1 cm of adult fish per litre of water. Use whichever unit you measured your tank in; the fish tank volume calculator gives both.

Does the rule apply to saltwater tanks? No. Marine fish need much more space per inch. Saltwater stocking is usually planned at a much lower density than freshwater.

Plan your stock around the real water volume, choose fish by their adult size, and add them slowly. Your tank will stay stable and your fish will thrive.