Answer
How many fish can I put in my tank?
The common starting rule is about 1 inch of adult fish per gallon of water, for small, slim fish only: a 20-gallon long holds about 14 gallons of water after gravel, so roughly 14 inches of small fish such as tetras. Large or messy fish need far more room.
By tank size
| Tank | Water after gravel | ≈ Inches of small fish |
|---|---|---|
| 5.5 gallon | 3.9 gal | 4 |
| 10 gallon | 7.8 gal | 8 |
| 15 gallon | 11.2 gal | 11 |
| 20 high | 16.2 gal | 16 |
| 20 long | 14 gal | 14 |
| 29 gallon | 23.4 gal | 23 |
| 40 breeder | 36.5 gal | 36 |
| 55 gallon | 48.6 gal | 49 |
| 75 gallon | 67.3 gal | 67 |
| 90 gallon | 78.5 gal | 79 |
| 125 gallon | 101 gal | 101 |
Where the rule fails
- Goldfish, cichlids and plecos produce much more waste per inch.
- Adult size counts, not the size at the store.
- Schooling fish need a group; territorial fish need space more than volume.
More questions
How many fish in a 10-gallon tank?
Roughly 8 inches of small fish, for example a small school of nano fish; goldfish need far more space.
Should I add all the fish at once?
No: add a few at a time so the filter’s bacteria can keep up.
How many fish in a 20-gallon tank?
Roughly 14 inches of small, slim fish in a 20 long after gravel, fewer for larger or messy fish.
How many fish in a 55-gallon tank?
Roughly 49 inches of small fish at most; plan by adult size and species.
How many goldfish per gallon?
Goldfish need far more than an inch per gallon because they grow large and produce lots of waste.
Does a bigger filter let me add more fish?
It helps with waste, but space, oxygen and behaviour still limit stocking.
Should I count the fish’s current size?
No, count its adult size.
How fast can I add fish?
A few at a time, so the filter’s bacteria can catch up.
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Sources
- Water volumes computed from standard tank dimensions (in³ ÷ 231). The inch-per-gallon rule is a long-standing hobby rule of thumb, not a scientific standard.