Betta tank size and tank mates
Most keepers start at 5 gal for one betta. The 5 gal preset holds 4.9 gal of water after substrate, which is 4.5 in of fish by the one-inch rule.
- 5.5 gal
- gross
- 4.9 gal
- net of substrate and decor
- 4.5 in
- by the inch rule
- 2.5 in
- one adult betta
4.9 gal net · max 1 fish · comfortable 1
most adult fish this tank holds by the rule
comfortable: 1 (80%), at 2.5 in each
- One-inch-per-gallon rule: 1 in of fish per 1 gal.
- 5.5 gal gross, minus 0.7 gal (12%) for substrate and decor = 4.9 gal of water.
- Surface-area check: 16 × 8 = 128 sq in, about 10.5 in of tropical fish at 12 sq in per inch.
- Under 5 gallons, water quality swings fast. Most keepers stock only a single betta or shrimp here.
A rule of thumb, not science. Adult size, temperament, swimming space, and each species' minimum tank matter more than any inch rule.
| Tank | Net gal | Max in, small fish | Betta alone | Betta + snails or shrimp | Betta + small community |
|---|---|---|---|---|---|
| 2.5 gal | 2.2 | 2 | Possible, but little margin | One snail, if any | No |
| 5 gal | 4.9 | 4.5 | Good starting size | A snail or a few shrimp | No |
| 10 gal | 9.1 | 9 | Comfortable | Yes | A small school of ember tetras or a few corydoras, if your betta allows |
| 20 gal long | 16.5 | 16 | Roomy | Yes | Yes: schooling fish and bottom dwellers with a hiding-place plan |
Guidance is what keepers commonly suggest. Each betta differs, and the inch rule counts the betta (2.5 in) as part of the total.
| Tank mate | Adult size | Caveats | Usual minimum tank |
|---|---|---|---|
| Nerite snail | 1 in | Usually ignored. Lids needed; they leave the water. Eggs will not hatch in freshwater. | 5 gal |
| Malaysian trumpet snail | 1 in | Breeds quickly, but is rarely harmed. Digs through substrate. | 5 gal |
| Cherry shrimp | 1.25 in | Some bettas eat them, especially young ones. Add heavy plant cover. | 5 gal |
| Ember tetra | 0.8 in | Keep 6+. Small and quick, so usually left alone. Not a pick for aggressive bettas. | 10 gal |
| Harlequin rasbora | 2 in | Keep 6+. Calm schooling fish. Their size is close to a betta's own. | 10 gal |
| Panda cory | 2 in | Keep 6+. Bottom dwellers that mostly stay out of the betta's zone. Use sand. | 10 gal, 20 long better |
| Otocinclus | 1.75 in | Keep 4+. Needs a mature tank with algae to graze. | 10 gal |
| Kuhli loach | 4 in | Keep 5+. Shy and nocturnal. Secure lids: they escape. | 20 gal long |
| Fish | Why |
|---|---|
| Other male bettas | They fight, sometimes to the death. |
| Fin-nippers: tiger barbs, some serpae tetras | They shred long fins, and a stressed betta gets sick. |
| Guppies and other fancy-tailed fish | Their color and fins can provoke a betta to attack. |
| Goldfish, angelfish, and other cichlids | Different temperature range or too aggressive and big. |
How to use this calculator
Start from 5 gal
The calculator is set to a 5 gal tank and a 2.5 in adult betta.
Add tank mates by inches
Change the fish size to a tank mate's adult size to see how many fit. The betta counts as one of the fish, so subtract 2.5 in.
Check the tank mate table
Rule math cannot tell you about temperament. The table lists caveats and a minimum tank for each.
The rule we use
For this page, the calculator applies the rule that matches the fish, and takes 12% of the gross gallons off by default for substrate and decor. The three rules are 1 gal per inch for small slim community fish, 3 gal per inch for goldfish and fish of 5 in or more, and 5 gal per inch for saltwater. See aquarium stocking rules explained for the formula, filtration factors, and the origin of each.
These are rules of thumb used by hobbyists, not science. The inch rule says nothing about temperament. Each betta differs: some live peacefully with others, some attack anything. Plan a way to separate them.
Common mistakes
Putting two male bettas together
They will fight, sometimes to the death. Dividers rarely fix it: bettas flare at each other through clear ones.
Adding fin-nippers or fancy-finned fish
Tiger barbs nip long fins. Guppies' fancy tails can provoke a betta. Neither is a good match.
Choosing a vase or a bowl
Under 5 gal, temperature and ammonia swing fast, and there is no room for a heater or a filter.
Frequently asked questions
What size tank does a betta need?
Most keepers recommend at least 5 gal with a heater and gentle filter. Larger tanks are easier to keep stable.
Can a betta live in a 2.5 gallon tank?
It can, but it leaves no room for error. Under 5 gal the water changes quickly, and there is room for a betta and nothing else.
Can I keep a betta with other fish?
Sometimes. Common tank mates are snails, shrimp, and small schooling fish such as ember tetras in 10+ gal. Each betta differs.
Will a betta eat shrimp?
Some do. Cherry shrimp may be eaten, especially young ones. Heavy plants and hiding places improve their odds.
How many bettas can I keep together?
One male per tank. Sorority tanks of females need 20+ gal and experienced care and still fail often.
How many ember tetras go with a betta in a 10 gallon?
A school of 6 is a typical choice. They total 4.8 in, and the betta adds 2.5 in, which is about 7.3 of 9 in of water capacity.