How many fish can I put in a 10, 20, 29, or 40 gallon tank?

Short answer: there is no exact formula, but as a rough starting point: a 10 gallon suits one betta or a single small school with shrimp; a 20 gallon holds a modest community of roughly fifteen to twenty small fish across two or three species; a 29 gallon gives room for a fuller community; and a 40 gallon comfortably holds two schools plus a centrepiece and a cleanup crew. Always stock for the adult size of each fish, and add them gradually.

The old rule of one inch of fish per gallon is only a loose guide. It ignores how much waste a fish makes, how big it grows, and how it behaves. A single fancy goldfish outgrows a 10 gallon, while twenty small tetras might not. Use the examples below as sensible, peaceful starting points, not hard limits.

10 gallon

A 10 gallon is a nano tank. It is happiest as a single-species setup.

• One betta, on its own or with a few snails.
• Or a single small school — for example six to eight nano rasboras — with a group of cherry shrimp.

Keep it simple. The small water volume swings quickly, so one modest group is safer than a crowd.

20 gallon

This is the size where a proper community becomes possible. One sensible combination:

• A school of eight to ten small tetras such as neons or ember tetras.
• A group of six corydoras for the bottom.
• A few snails as a cleanup crew.

Add each group over several weeks rather than all at once, so the filter can keep pace.

29 gallon

The extra height and footprint let you add a centrepiece or a second small school:

• Ten to twelve tetras or harlequin rasboras.
• Six to eight corydoras.
• A small, peaceful centrepiece such as a honey gourami, plus shrimp or snails.

40 gallon

A 40 gallon, especially a breeder shape, is a genuinely comfortable community tank.

• Two schools, for example a dozen tetras and a second group of rasboras.
• A group of eight corydoras.
• A centrepiece pair, plus a shrimp and snail cleanup crew.

Even here, resist filling every space at once. A lightly stocked tank is more stable, clearer, and easier to keep.

Stocking for Southern Alberta water

Our tap water is generally hard, which suits guppies and other livebearers especially well — a good choice for a first community. You can browse everything in the freshwater fish collection. Whatever you pick, check that the species are peaceful together and want similar water before you buy. We are always glad to confirm a combination.

New to all this? Start with your first aquarium, and make sure your filter can handle the bioload you are planning.

Written by the team at Tropical Fish Imports in Lethbridge, Alberta. Last reviewed August 2026.

Want a second opinion on a stocking plan before you commit? Tell us your tank size and we will help you build a combination that stays peaceful as the fish grow — message us or visit the shop in Lethbridge.