How many fish in a 40 gallon tank?
A pair of dwarf cichlids with a school of tetras, or two fancy goldfish.
The usual 40 gallon is the breeder, 36 inches long and a roomy 18 inches front to back. That width is what makes it a favorite: bottom fish and cichlids get real floor space.
Stockings that work in a 40 gallon tank
Each mix has been run through the stocking calculator and comes out with no problems or warnings.
- Bolivian rams with rummy-nose tetras50% stocked
2 × Bolivian ram, 12 × Rummy-nose tetra.
- Two fancy goldfish81% stocked
2 × Fancy goldfish.
- Cherry barbs, kuhli loaches and honey gouramis89% stocked
8 × Cherry barb, 6 × Kuhli loach, 4 × Honey gourami.
- A big school of neon tetras with corydoras79% stocked
20 × Neon tetra, 8 × Bronze cory.
- Platies, mollies and a bristlenose pleco59% stocked
6 × Platy, 4 × Molly, 1 × Bristlenose pleco.
Why not one inch of fish per gallon
That old rule says to allow one inch of adult fish per gallon. It is easy to remember and works well for small, slim fish like tetras and rasboras.
It breaks down for bigger and messier fish. A fish twice as long weighs several times as much, and goldfish and plecos make far more waste than their length suggests. Our calculator keeps the inch rule for fish under three inches and counts larger fish for more.
The 36 x 18 inch footprint meets the Seriously Fish minimum for Bolivian rams, kribensis and yellow labs, which a 29 or 55 does not, and it is the smallest common tank that the goldfish guides allow two fancy goldfish in.
One kind of fish, nothing else
The most of each that suits a 40 gallon breeder on its own.
| Neon tetra | up to 50 |
|---|---|
| Guppy | up to 25 |
| Harlequin rasbora | up to 33 |
| Cherry barb | up to 30 |
| Platy | up to 30 |
| Panda cory | up to 30 |
| Bronze cory | up to 20 |
| Honey gourami | up to 6 |
| Pearl gourami | up to 4 |
| Tiger barb | up to 25 |
| Angelfish | up to 5 * |
| Fancy goldfish | up to 2 |
| Bristlenose pleco | 1 only |
| Electric yellow lab | up to 10 |
* Fits by volume, but the footprint is under what Seriously Fish recommends for the species.
Before you add the last fish
- Add fish slowly. A new tank needs weeks to grow the bacteria that process waste, so add a few fish at a time.
- Count the minimum group. Schooling fish are stressed alone, so plan around the smallest group before the tank total.
- Plan for adult size. Fish in the shop are usually young.
- Match the filter. Look for one rated for 160 to 240 gallons per hour.
A full tank is not a target. One at 60 to 80% is easier to keep clean and gives fish room to settle.
Sizes and weight: 40 gallon breeder tank dimensions.
Common questions
How many fish can live in a 40 gallon tank?
A pair of dwarf cichlids with a school of tetras, or two fancy goldfish. The example mixes on this page come out between 50% and 89% stocked by our calculator, with no warnings.
How many neon tetras fit in a 40 gallon tank?
Up to 50 neon tetras in a 40 gallon breeder with nothing else in it. Add other species and that number comes down.
Is one inch of fish per gallon a good rule?
It works for slim fish under about three inches, such as tetras and rasboras. It fails for larger or messier fish, because a fish twice as long has several times the body mass. Count big fish for more.
What size filter does a 40 gallon tank need?
One rated for 160 to 240 gallons per hour, which turns the water over four to six times an hour.
Gear for a 40 gallon breeder tank
Searches on Amazon for the right size. Check the tank size or flow rating on the listing before you buy.
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