TankStockingHow many fish in a 3.5 gallon tank?
A 3.5-gallon (13 L) tank is below the safe minimum for any fish in our database — it is a shrimp, snail, or planted display tank, not a fish tank.
It is tempting to put a betta or a few tiny fish in a 3.5-gallon, and most calculators will happily green-light it. We will not. At 13 litres the water chemistry swings too fast and there is no room for a fish to behave naturally, so every fish in our database — including the betta — trips the minimum-tank welfare check at this size. The honest options are a shrimp colony, a few snails, or a planted aquascape with no fish at all.
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Verdict
What fits a 3.5-gallon tank?
Each example below is scored by the real engine — including the ones it flags, to show you why.
Why a betta does not fit
✕ Not recommended1× Betta (Siamese Fighting Fish)
Even a single betta — the usual "nano" pick — is flagged here: a betta needs at least 5 gallons, heated and filtered. This is what an honest calculator should tell you.
Load this build in the planner ↑Even shrimp want more
✕ Not recommended8× Cherry Shrimp
A cherry-shrimp colony is the most realistic 3.5-gallon livestock, but our sourced minimum for them is 5 gallons of stable water — so plan for a 5-gallon and let the colony thrive.
Load this build in the planner ↑How many of each popular fish fit a 3.5-gallon tank?
The honest, engine-derived answer instead of a single guess: comfortable single-species display counts for popular community fish at this size. Each number is deliberately conservative — it leaves headroom for water-quality swings and tankmates, so it is a comfortable target, not a hard ceiling. Tap a count to load that fish in the planner.
| Species | Adult size | Comfortable count (this tank) |
|---|---|---|
| Neon Tetra | 3 cm | needs ≥ 10 gal |
| Cardinal Tetra | 3 cm | needs ≥ 15 gal |
| Ember Tetra | 2 cm | needs ≥ 10 gal |
| Harlequin Rasbora | 4.5 cm | needs ≥ 15 gal |
| Zebra Danio | 5 cm | needs ≥ 10 gal |
| Cherry Barb | 5 cm | needs ≥ 15 gal |
| Guppy (Fancy) | 5 cm | needs ≥ 10 gal |
| Platy | 6 cm | needs ≥ 10 gal |
| Molly (Common / Sailfin) | 12 cm | needs ≥ 20 gal |
| Bronze Corydoras | 7 cm | needs ≥ 20 gal |
| Kuhli Loach | 9 cm | needs ≥ 20 gal |
| Cherry Shrimp | 3 cm | needs ≥ 5 gal |
Good to know
What is the maximum number of fish for a 3.5-gallon tank?
For a 3.5-gallon tank the honest answer is no fish — it is below the sourced minimum tank size for every fish in our database, including the betta. It suits a shrimp colony, a few snails, or a planted aquascape only. For your first fish, step up to at least a 5-gallon.
Can I use the "one inch of fish per gallon" rule?
It is a rough starting point at best and breaks down quickly: a 3-inch goldfish produces far more waste than three 1-inch tetras, and the rule ignores schooling needs, aggression, and adult size. TankStocking weights bio-load by body size and waste class and applies hard welfare checks instead.
Should I add all the fish at once?
No. Cycle the tank first, then add fish in small batches over several weeks so the biological filter can keep up. A fully-stocked plan is the destination, not the starting point.
Plan a 3.5-gallon tank
Related guides on TankStocking — each scored by the same welfare engine as the planner.
Tank-mate guides for these fish
Stocking levels are planning estimates, not guarantees — individual fish, filtration, planting, and maintenance all matter. Cycle the tank before adding livestock and verify your own water. How TankStocking works →