FishBaseHQ

2026-10-28 · 10 min read

How many fish can fit in a 20-gallon tank?

A 20 gallon long can hold a tetra school and corydoras. A 20 high is worse for swimming. Inch-per-gallon is not the answer — adult size, waste, and filter GPH are.

“How many fish in a 20-gallon?” is the wrong question. The right ones: 20 long or 20 high?, what species as adults?, what filter?

A 20 long is 30×12×12 inches. That length is why it is the default first community. A 20 high is the same nickname gallons and a worse swim — tetras pace a hallway, not a stairwell. Confirm the water you actually have: volume.

A 20 long that works

Cycled. Hang-on-back rated well above 80 GPH (buy closer to 150–200 GPH on the box; empty-filter numbers lie). Sand. A heater at 74–78°F.

MixCountNotes
Neon or ember tetras10–12One school. Not two competing midwater species in this footprint.
Corydoras6Same species, sand, sinking food.
Optional later0–1 bristlenoseCave, wood, wafers. A 29 is more honest if you already have the school + corys.

That is a community. It is not a mall tank. Neon tetras, harlequin rasboras, and corydoras are the usual pieces. Swap the tetras for rasboras if you want; do not stack both at 12 each.

Stay under about 90% on the stocking calculator if you are new. The tool uses adult size, a waste factor, and turnover. It does not use inch-per-gallon.

A 20 that does not work

  • Goldfish. One fancy goldfish is already an argument; two is a waste factory. See fancy goldfish.
  • Oscars, Jack Dempseys, mbuna. Aggressive list.
  • Common pleco “for algae.” Common pleco.
  • A betta plus a tetra school plus corys plus shrimp. That is four ideas. Pick two at most, and read the betta checker before you try.

Inch per gallon

One inch of fish per gallon ignores body mass, goldfish guts, and whether the filter is a toy. A 6-inch goldfish is not six neons. We do not use the rule. If a site still quotes it, they are counting inches instead of ammonia.

Order of operations

  1. Cycle. Clear water is not a cycle. Cycling.
  2. Compatibility on the whole list at 20 gallons.
  3. Stocking with adult inches and the GPH you will actually buy.
  4. Add the school first. Wait two weeks. Add the corys. Wait again.

The community planner is that list in one place. If the 20 is still empty, do not shop livestock this weekend.

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