Tetra Fish: Tank Size, Group Size, Tank Mates, Lifespan

A tetra is a small, usually peaceful schooling fish from the rivers of South and Central America or Africa, and the name covers many species rather than one. Wikipedia explains that 'tetra' comes from Tetragonopterus, an old genus name meaning square-finned, and is no longer a scientific grouping: tetras now sit in seven families.

Most share a small, fleshy adipose fin between the dorsal fin and the tail, a feature worth checking when you are trying to tell a tetra from a similar-looking rasbora or barb. The best known is the neon tetra, Paracheirodon innesi, from the western and northern Amazon basin. Wikipedia reports that about 2 million are sold in the US each month.

One naming trap: Tetra is also a large brand of fish food and aquarium products, so search results for 'tetra fish' often mix the fish with the brand.

Keep at least six of the same species, and more is better. The sources agree on the floor and differ only on how far above it to go:

• PetMD: at least five to six; larger groups reduce stress
• LiveAquaria: six or more of the same species
• Wikipedia (neon tetra): at least six, and more active in groups of eight to 12 or more
• Seriously Fish (neon tetra): a mixed-sex group of at least 8 to 10

Mixing three neons with three cardinals does not make a school of six. Each species schools with its own kind, so buy in species groups.

For small species such as neons, plan on at least 10 gallons, and more for a proper school. PetMD gives 10 gallons as the minimum for neons and suggests a rule of thumb of one gallon per inch of fish. Seriously Fish recommends a tank base of at least 60 by 30 cm, about 54 litres or 14 US gallons, for a neon group.

For very small tanks Aqueon is stricter: in 5 to 10 gallons it suggests diamond-shaped tetras such as serpae or black phantom in groups of 4 to 6, and advises against neon or cardinal schools in this size unless you keep 12 or more, with no other tankmates besides maybe some corydoras or a bristlenose pleco. Larger tetras such as the Congo tetra need much more room; PetMD's size range for tetras runs up to 5 inches.

Most tetras are not hard to keep, but the most popular one, the neon, has problems that start before you buy it. Seriously Fish says years of commercial breeding have weakened the ornamental neon strain, so deformities and ill-health are widespread. Neons are also prone to neon tetra disease, a parasite infection that Wikipedia calls generally incurable and often fatal.

Difficulty varies a lot by species. On LiveAquaria's tetra page (checked 25 September 2026), the neon, black neon, X-ray and red eye tetras are rated Easy, while the cardinal tetra and brilliant rummynose tetra are rated Expert Only. PetMD calls tetras beginner to intermediate depending on species.

In practice: if you are new, start with a species rated Easy, keep the water within that species' range, and separate any fish that loses colour or develops lumps before it affects the rest of the group.

Choose calm fish of similar size. PetMD lists danios, rasboras, cory catfish, gouramis, guppies, mollies, platies, swordtails, hatchetfish and rainbowfish as good companions. Seriously Fish adds similarly sized characins, smaller catfish and non-predatory cichlids for neons, and notes the neon feels more secure alongside other schooling fish.

The practical limit is mouth size. A neon is only about 30 mm long without the tail (Seriously Fish), so any fish big enough to swallow it is a risk, however peaceful it looks in the shop. A single betta can share a large enough tank with a neon group; see our betta fish page for how much space that takes.

Retail prices on one day at two online shops. Listings may be for different quantities, so check how many fish a price buys:

• Flip Aquatics: ember tetra $5.99, neon tetra $6.99, green neon tetra $7.99, cardinal tetra $8.99
• LiveAquaria: black neon tetra from $21.99, neon tetra from $24.99, cardinal tetra from $29.99, Congo tetra $32.99, GloFish tetras from $39.99

Neon and cardinal look alike. Wikipedia's way to tell them apart: the cardinal's red extends much further along the body.