A 20-gallon tank (about 75 litres) is the sweet spot for many hobbyists. It is large enough for a proper school, a showpiece fish, and a bottom crew, yet still cheap to heat and easy to keep stable. The extra water volume also forgives the small mistakes that sink a 10-gallon setup.
Why the 20-gallon size works
Compared with a 10-gallon, a 20-gallon tank doubles your stocking headroom and stabilises temperature and chemistry far better. You can keep a centre fish that would terrorise a smaller tank, run a bigger plant mass, and still leave room to breathe. Start by reading the nitrogen cycle so the tank is ready before fish arrive.
Top schooling fish for a 20-gallon
- Neon tetras — A classic 10+ school; needs stable, warm, soft water.
- Cardinal tetras — Like neons but with a fuller red stripe; keep a group of eight or more.
- Ember tetras — Tiny glowing nano-school that pops against green plants.
- Rummy nose tetras — Peaceful; their colour tells you when water quality dips.
- Harlequin rasboras — Tough, peaceful, and happy in a planted 20-gallon.
- Celestial pearl danios — Speckled nano fish that use all levels of the tank.
- Cherry barbs — Colourful, active, and more robust than their looks suggest.
Centerpiece fish
- Dwarf gourami — A single specimen adds colour and personality without bullying.
- Ram cichlid — A small, peaceful cichlid that rewards clean, warm water.
Bottom dwellers and the algae crew
- Corydoras catfish — Keep a group of six on sand for endless bottom activity.
- Pygmy corydoras — A miniature cory that shoals in the mid-water too.
- Otocinclus — Small suckermouth catfish that graze soft algae off leaves.
- Bristlenose pleco — One adult keeps driftwood and glass tidy; stay under one per tank.
- Cherry shrimp and amano shrimp — A shrimp colony cleans where fish cannot reach.
- Nerite snails — Algae-eating snails that will not breed in freshwater.
Livebearers if you want movement and colour
Guppies, platies, mollies, swordtails and endlers all do well in a 20-gallon tank. Bear in mind livebearers breed fast, so a mixed tank can fill up quicker than you expect — compare with a 10-gallon stocking plan if you are unsure.
Fish to avoid in 20 gallons
- Goldfish — they need 20+ gallons each and produce heavy waste.
- Common plecos — they reach 30 cm and quickly outgrow the tank.
- Angelfish — a mature pair needs more length than a 20 offers.
- Most African cichlids and large barbs — too territorial or too big.
A sample 20-gallon stocking plan
One dwarf gourami, a school of 10 neon or cardinal tetras, six corydoras on sand, a small group of cherry shrimp, and two nerite snails. That is a balanced, active community with three water levels covered and a light cleanup crew.
Before you buy
Cycle the tank fully, then add fish gradually over a few weeks so the filter keeps up. Keep up regular water changes and lean on the beginner setup guide if you are building the tank from scratch. A planted tank also steadies water and gives shy fish cover.
