How many fish can I put in a 10, 20, 29 or 55 gallon tank?
Short answer: as a planning baseline, a 10-gallon tank holds about 8–12 small (under-2-inch) fish, a 20-gallon about 15–20, a 29-gallon about 20–25, and a 55-gallon about 30–40 — fewer if you keep larger or messy species. These are limits for slim community fish with good filtration, not hard maximums. Body size matters more than fish count: one 6-inch fish makes more waste than six 1-inch fish.
Stocking is the single most common beginner mistake. Below are realistic example stocks for the four most popular tank sizes, why the old "inch per gallon" rule misleads, and a calculator that does the math for your exact fish.
Why "1 inch of fish per gallon" misleads you
The classic rule says you can keep one inch of adult fish length per gallon of water. It is easy to remember and roughly okay for small, slim fish — but it falls apart fast:
- Waste scales with body mass, not length. A fish twice as long is also taller and wider, so it can weigh many times more and produce far more waste. Six 1-inch tetras and one 6-inch fish are not equivalent loads.
- It ignores shape. A slim danio and a deep-bodied goldfish of the same length have very different bioloads.
- It ignores surface area. A long, shallow tank exchanges more oxygen than a tall, narrow one of the same volume.
Our stocking calculator weights larger fish more heavily and factors in tank volume as a stand-in for surface area, so it gives a more realistic reading than the raw inch rule.
How many fish fit in each tank size
10 gallon tank
A 10-gallon is a nano tank. Good options:
- 1 male betta + a few snails or a small shrimp colony
- A school of 6–8 ember tetras, chili rasboras, or neon tetras
- 5–6 guppies or endlers (mostly males to avoid a population boom)
Avoid anything that grows past ~2 inches or schools in large numbers.
20 gallon tank
The classic first community tank. A balanced stock:
- 8–10 harlequin rasboras or neon tetras (a school)
- 6 corydoras catfish (bottom level)
- 1 dwarf gourami or a pair of honey gouramis (centerpiece)
29 gallon tank
The extra height and footprint let you run two levels comfortably:
- 10 rummy-nose or glowlight tetras
- 8 corydoras or a small group of kuhli loaches
- A pair of honey gouramis or a single pearl gourami
- An otocinclus group or a bristlenose pleco for algae
55 gallon tank
A 55 opens up bigger fish and fuller schools:
- 12 tetras + 6 corydoras + a pair of angelfish + a bristlenose pleco, or
- A school of rainbowfish with a pleco and corydoras, or
- A small group of medium cichlids (research compatibility carefully)
| Tank size | Small fish (under 2") | Example centerpiece |
|---|---|---|
| 10 gallon | ~8–12 | 1 betta |
| 20 gallon | ~15–20 | 1 dwarf gourami |
| 29 gallon | ~20–25 | 1 pearl gourami |
| 55 gallon | ~30–40 | pair of angelfish |
How to stock a tank without overdoing it
- Start with the tank, not the fish. Pick species that fit the size you actually have.
- Check adult size, not store size. Most fish are sold as juveniles.
- Respect schooling minimums. Tetras, rasboras, and corydoras need groups of 6+ to feel secure.
- Run the numbers. Drop your list into the stocking calculator and aim for 100% or below.
- Add fish gradually so your filter's bacteria can keep up, and test your water.
Stocking lightly still needs solid filtration. See our filter size calculator for the right GPH.
Frequently asked questions
How many fish in a 10 gallon tank?
About 8–12 small fish — one betta, or a single school of 6–8 nano fish, or 5–6 guppies. Keep it lightly stocked.
How many fish in a 20 gallon tank?
Around 15–20 small fish: a school of 8–10, six corydoras, and a centerpiece fish is a balanced 20-gallon community.
How many fish in a 29 gallon tank?
Roughly 20–25 small fish — two schools plus a centerpiece pair and an algae-eater.
How many fish in a 55 gallon tank?
About 30–40 small fish, or fewer medium fish such as a pair of angelfish with schools and a pleco.