Best Fish for a 20-Gallon Tank

A 20-gallon (75-litre) tank is one of the most versatile sizes in the hobby. Here are the fish that genuinely thrive in it — and the ones that don't.

A 20-gallon (about 76 litres) tank is widely recommended for beginners because it balances stability with size. But "20 gallons" does not mean "any fish" — the species must fit the floor space, water and temperament of a tank this size.

Good choices

Fish	Why it suits a 20-gallon
Betta (single)	Comfortable size for one fish, with room for a few snails or shrimp
Guppies / platies	Hardy, colourful livebearers that use the whole tank
A shoal of small tetras	Neons, glowlights or harlequins in a group of six or more
Corydoras	A group of six small corys for the bottom
A pair of small cichlids	Bolivian rams or kribensis, if the tank is planned around them

How to stock it

Aim for one or two shoaling species plus a bottom-dweller, rather than a bit of everything. Add fish slowly, a group at a time, and let the filter catch up between additions. Use the stocking calculator as a rough check, not a target to fill.

What to avoid

Fast-growing large fish — common plecos, oscars, clown loaches and anything that will outgrow the tank.
Large, aggressive cichlids that claim the whole footprint.
Fish that need length — danios, rainbowfish and scissortails want a longer tank than a 20-gallon usually is.
Many species at once — a mix of half-groups looks busy and stresses the fish.

Setting it up well

Cycle the filter before adding fish, keep the tank out of direct sun, and match the décor to the species. A planted tank with soft, easy plants is a strong choice at this size. See the full 20-gallon setup guide.

Why 20 gallons is a sweet spot

A 20-gallon tank is big enough to hold a real community, forgiving enough for a beginner, and small enough to fit on a desk or shelf. Its water is more stable than a 5- or 10-gallon, so small mistakes do not turn into disasters, but it is still a modest weekly water change. The catch is that "fish that fit" and "fish that thrive" are different things — many species sold as small grow far too big.

A centrepiece fish

Betta — one male makes a stunning centrepiece; no tankmates that nip fins.
Honey or dwarf gourami — a calm, colourful single or pair.
Apistogramma — a pair of these dwarf cichlids is full of character.
Bolivian ram — peaceful, hardy and long-lived.

Schooling fish

Small schooling species look best in groups of six or more and swim in the upper and middle water. Good choices include neon and cardinal tetras, glowlight tetras, harlequin rasboras, celestial pearl danios, chili rasboras and ember tetras. Choose one or two species and give each a proper group, rather than a lonely single of several kinds — shoaling fish are calmer and show better behaviour in numbers.

Bottom dwellers

Corydoras catfish are the classic bottom crew and do best in groups of six or more — panda, pygmy, bronze and peppered corydoras all suit a 20-gallon well. They need a soft, smooth substrate so their barbels are not injured, and are far happier on sand than sharp gravel. A single bristlenose pleco can work if you want an algae grazer, but most other plecos grow far too large.

Shrimp and snails

Shrimp and snails add life and clean-up without much waste. Neocaridina shrimp, amano shrimp, nerite snails and mystery snails all suit a 20-gallon. Check compatibility, though — some fish eat shrimp, and some snails reproduce enthusiastically. They are best in a planted tank with plenty of cover.

What to avoid

Goldfish — grow huge and produce far too much waste.
Common plecos — reach 30cm or more.
Large cichlids — too big and often aggressive.
Bala sharks, tinfoil barbs, iridescent sharks — all outgrow the tank dramatically.

Planning the stock

A simple, balanced 20-gallon might hold one centrepiece fish, a group of six to eight small schooling fish and six corydoras, plus a few shrimp. Build up slowly so the filter's bacteria keep pace, and check the adult size, water needs and temperament of every fish before buying. Chosen well, a 20-gallon community is one of the most rewarding tanks a beginner can run.

How many fish can a 20-gallon hold?

It depends on species. A community of small shoaling fish can number a dozen or more; a pair of cichlids may fill it. Match fish to tank.

Can I keep a betta with other fish in 20 gallons?

Sometimes — with peaceful mid-water fish and no fin-nippers. Every betta differs, so watch for aggression.

Is 20 gallons too small for corydoras?

No — a group of six small corys (panda, pygmy) suits it well, with soft substrate.

Tools for this guide

Sources and research notes

Stocking advice follows standard fishkeeping guidance for small tanks; exact numbers depend on species, filtration and routine.

Written and edited by the Tankwise Editorial. Every guide answers one real fishkeeping question, is checked against the sources listed above, and is updated when guidance changes. We are an independent editorial desk, not veterinary or aquarium professionals — read how we research and update our guides.