Small Aquarium Fish: What Fits 3, 5 and 10 Gallon Tanks

The best small aquarium fish stay under about 2 inches as adults and are happy in a single-species group, such as chili rasboras, ember or green neon tetras, white cloud minnows, Endler livebearers or a single male betta. Aqueon's rule for 5 to 10 gallon tanks is fish between ½ and 2 inches at most, and Aquarium Science recommends keeping only one species in any tank under 10 gallons.

Sizes of the fish most often suggested, from the sources we checked:

• Chili rasbora, Boraras brigittae: 10 to 20 mm, from peat swamps in southwest Borneo (Wikipedia)
• Green neon tetra: about 1 inch, smaller than a regular neon (Aquarium Co-op)
• Rosy loach: 1.25 inches, kept in a school of 6 or more (Aquarium Co-op)
• Dwarf pea puffer, Carinotetraodon travancoricus: up to 3.5 cm, usually under 2.5 cm (Wikipedia)
• Sparkling gourami, Trichopsis pumila: up to 4 cm, needs 25 C or warmer (Wikipedia)
• White cloud mountain minnow, Tanichthys albonubes: about 4 cm, likes 18 to 22 C (Wikipedia)
• Betta, one male alone: see our betta fish page

Aquarium Science also notes that aggression shows up easily in a small tank and that good filtration and aeration matter more, not less, as the tank gets smaller.

Under 5 gallons, plan on a single betta or one very small single-species group. Aquarium Science's list for tanks under five gallons is: a male betta kept alone, Endler livebearers, scarlet badis, dwarf chain loaches, peacock gudgeons, sparkling gouramis, or Tanganyikan shell-dwelling cichlids that need empty snail shells to live in. Pick one of these, not several.

Some common fish do not fit at all. PetMD's guppy guide gives 5 gallons as the minimum for a single adult guppy, so a 3 or 4 gallon tank is below that minimum even for one guppy. Goldfish, plecos and other fish that grow large do not belong in a tank this size.

Aquarium Co-op's five picks for a 5-gallon tank are one dwarf pea puffer, a school of 6 or more rosy loaches, a school of 6 green neon tetras, a group of white cloud mountain minnows, or one scarlet badis. It adds two cautions: pea puffers and scarlet badis will eat dwarf shrimp, and pea puffers also hunt snails.

One point where sources disagree: Aquarium Co-op advises only one pea puffer in 5 gallons because they are territorial, while Wikipedia describes wild dwarf puffers as highly social fish that form large shoals and become stressed when kept alone. Both can be true. A shoal needs far more space than 5 gallons, so in a small tank a single puffer is the realistic option.

White clouds are the one choice that Aquarium Co-op says does not even need a heater; Wikipedia gives their natural range as 18 to 22 C. Every other fish on this page needs a heater.

A 7-gallon tank falls in Aqueon's 5 to 10 gallon band, which allows one group of small fish: diamond-shaped tetras such as serpae or black phantom in groups of 4 to 6, livebearers (guppies, platies, common mollies) as at least 3 fish with 1 male to 2 or 3 females, a small group of zebra danios, or 4 to 6 small corydoras such as panda or salt-and-pepper corys. Aqueon says a tank this size usually holds only one main group.

The sources are not identical here. PetMD's guppy formula, 5 gallons for the first adult plus 2 for each additional fish, works out to only two guppies in 7 gallons, fewer than Aqueon's minimum of three livebearers. When sources disagree, stock at the lower number; it is easier to keep a lightly stocked small tank stable.

• One species under 10 gallons (Aquarium Science)
• Heater for tropical fish: sparkling gouramis need 25 C or warmer and dwarf puffers 22 to 28 C (Wikipedia); only white clouds prefer it cooler
• Filtration and aeration: Aquarium Science ties any heavier stocking to very good biofiltration and aeration
• Live plants: Aquarium Co-op recommends them in nano tanks for shelter and exploring
• Quarantine new fish: Flip Aquatics says it holds its fish two weeks or more before selling them; at home, a separate tank for new arrivals does the same job
• Check adult size before buying: a fish that is 1 inch in the shop may not stay that way. Photograph an unlabelled fish and the FishLens app identifies it

Prices per listing on Flip Aquatics' nano fish page on one day:

• Ember tetra $5.99
• Neon tetra, cherry barb or harlequin rasbora $6.99
• Green neon tetra or exclamation point rasbora $7.99
• Dwarf emerald rasbora or otocinclus catfish $8.99
• Endler livebearers from $8.99
• Pygmy corydoras $10.99

Prices change often and differ between shops; use them as a rough guide to what a small school costs.