Saltwater tank stocking
Saltwater uses 1 in of fish per 5 gal of water. A 40 gal breeder has 39.5 gal after rock and sand, so 7.5 in of fish.
- 44.9 gal
- gross
- 39.5 gal
- net of substrate and decor
- 7.5 in
- of fish, 5 gal per inch
- 2
- ocellaris clownfish, max
39.5 gal net · max 2 fish · comfortable 2
most adult fish this tank holds by the rule
comfortable: 2 (80%), at 3 in each
- Saltwater rule: 1 in of fish per 5 gal.
- 44.9 gal gross, minus 5.4 gal (12%) for substrate and decor = 39.5 gal of water.
A rule of thumb, not science. Adult size, temperament, swimming space, and each species' minimum tank matter more than any inch rule.
| Tank | Gross gal | Net gal (12%) | Max in | 3 in fish |
|---|---|---|---|---|
| 10 gal | 10.4 | 9.1 | 1.5 | 0 |
| 20 gal high | 19.9 | 17.6 | 3.5 | 1 |
| 20 gal long | 18.7 | 16.5 | 3 | 1 |
| 29 gal | 28.1 | 24.7 | 4.5 | 1 |
| 40 gal breeder | 44.9 | 39.5 | 7.5 | 2 |
| 55 gal | 56.7 | 49.9 | 9.5 | 3 |
| 75 gal | 78.5 | 69.1 | 13.5 | 4 |
| Fish | Adult size | Suggested minimum tank | Count in the 40 breeder, by rule | Notes |
|---|---|---|---|---|
| Ocellaris clownfish | 3 to 4 in | 20 gal | 2 | Often kept as a pair. Hardy and a common first marine fish. |
| Royal gramma | 3 in | 30 gal | 2 | Can be territorial with similar-looking fish. Needs caves. |
| Firefish | 3 in | 20 gal | 2 | Jumps. Use a tight lid. Shy without hiding spots. |
| Banggai cardinal | 3 in | 20 gal | 2 | Peaceful. Often kept in a pair. Buy captive-bred. |
| Six line wrasse | 3 in | 30 gal | 2 | Can be aggressive to similar fish. Good for pest control, but a poor mate for shy fish. |
| Yellow watchman goby | 4 in | 20 gal | 1 | Sand sifter that pairs with a pistol shrimp. Jumps. |
Suggested minimums are common hobbyist guidance and vary by source. Counts use the larger end of each size range and treat each species alone. Add marine fish one at a time, weeks apart, and research compatibility first.
How to use this calculator
Enter your tank
Use the net water volume. Saltwater tanks with live rock lose more volume, so raise the deduction to 15%.
Pick the adult size
Most beginner marine fish top out at 3 to 4 in. Choose medium or enter the exact size.
Check each species
Use the species table for a suggested minimum tank. Some small fish need 20 or 30 gal for behavior, not for inches.
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 5 gal per inch rule is a conservative saltwater convention. It is a ceiling, and stable chemistry matters more than the number.
Common mistakes
Stocking quickly
A saltwater tank needs a longer cycle. Add one fish at a time over months.
Choosing by the label, not the adult size
A tiny tang is a large fish later and needs a big tank. Check the adult size first.
Mixing aggressive fish in small tanks
Six line wrasses and some other small fish can be aggressive despite their size.
Frequently asked questions
How many fish in a 40 gallon saltwater tank?
7.5 in of fish by the 5 gal per inch rule: about 2 fish at 3 in. Most keepers stock fewer.
Is the 1 inch per 5 gallon rule right?
It is a common saltwater rule of thumb. It is conservative on purpose, because marine fish tolerate water changes less well.
How many clownfish in a 20 gallon?
A pair of ocellaris is commonly kept in 20 gal, but the 5 gal per inch rule allows only about 3 to 3.5 in of fish there, so a pair of 3 in fish is already above it. Many keepers choose 29 gal or more for a pair.
Do corals and inverts count?
Not in this rule. They add bioload, though, so stock fewer fish in a mixed reef.
Why is a nano reef harder?
Under 20 gal, chemistry shifts quickly. Keep salinity and temperature stable.