Description
Heterandria formosa, the least killifish, is the smallest known livebearer and one of the smallest fish in the world: males reach 2 cm and females about 3.6. It is a tiny fish from the United States that lives in the heavily vegetated margins of still or slow-moving waters, and it is recognised on sight by the gonopodium, enormous for such a body.
It is peaceful, but it is not a community fish in the ordinary sense: almost any commonly kept tankmate sees a male as a snack. It breeds with great ease and in a way of its own: the female drops a few fry every other day, not broods. It must not be confused with the mosquitofish (Gambusia), far more aggressive and also called mosquito fish.
Behavior
Very peaceful and not very competitive: it does not stand up to vigorous tankmates and loses out on food. Males may show a little aggression towards each other over the females, but physical damage is very rare. It works best in a small colony, with several of each sex.
It lives well with species of similar size and not predatory, such as pencil fish, dwarf corydoras, otocinclus and freshwater shrimp.
Avoid housing it with almost any ordinary community fish, which will eat the males, and with vigorous companions that take its food.
Habitat
Heterandria formosa lives only in the United States, from the Cape Fear River drainage in North Carolina to southern Louisiana; the type locality lies between Charleston, South Carolina, and Palatka, Florida. It occupies the shallow, heavily vegetated margins of still or sluggish waters, and in some places also occurs in brackish water.
That is the key to the set-up: barely moving water, abundant plants and a gentle filter. Current bothers it.
Details
🌊 Water parameters
🐠 Minimum tank size
Breeding
Very easy: if there are males and females, it is almost impossible to stop it breeding. Gestation lasts about four weeks, and the species has a method of its own: superfoetation, which lets fry at different stages of development be present in the uterus at once. That is why the young are not born in defined broods, but one by one, a few every day or two.
The embryos take much of their nourishment through organs that work like the placenta of mammals, because the yolk is poor. The young are large at birth, accept brine shrimp nauplii and powdered food from day one, and the adults do not usually harm them. A heavily planted tank is needed.
Diet
Omnivorous and undemanding: it accepts almost any food, although it is especially fond of small prey, live or frozen. It also grazes on algae, so it is best to give it some plant matter.
- Staple: fine flakes or pellets, with crushed spirulina if there are no algae.
- Supplement: brine shrimp and daphnia, live or frozen, in good proportion.
What about your tank?
Enter your tank's values to see only the compatible species that fit them. Litres apply to fish.
Compatible fish
General guidance based on water parameters, behaviour and known exceptions; always review the individual animals' and aquarium's specific needs.
They share water: pH 7 and 24–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–8 and 23–26 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7 and 23–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7 and 23–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–8 and 26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7 and 24–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–8 and 21–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–8 and 24–26 °C.
They share water: pH 7–7.5 and 25–26 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7–8 and 24–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–7.5 and 20–25 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7–7.5 and 20–24 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–7.5 and 25–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7–7.5 and 24–26 °C.
They share water: pH 7 and 24–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7.5–8 and 24–26 °C.
They share water: pH 7.5–8 and 24–26 °C.
They share water: pH 7 and 20–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.2 and 22–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–8 and 20–22 °C.
They share water: pH 7–8 and 20–24 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7 and 23–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–8 and 21–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–7.2 and 22–26 °C.
They share water: pH 7 and 26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7 and 22–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7 and 23–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–8 and 23–26 °C.
They share water: pH 7–7.5 and 23–26 °C.
They share water: pH 7–8 and 24–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 23–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–8 and 24–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–8 and 22–26 °C.
Compatible invertebrates
General guidance based on water parameters, behaviour and known exceptions; always review the individual animals' and aquarium's specific needs.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–8 and 20–25 °C.
They share water: pH 7.5–8 and 24–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7.5–8 and 26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–7.5 and 20–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–8 and 20–26 °C.
They share water: pH 7–8 and 20–26 °C.
Sources
- FishBaseFroese, R. and D. Pauly. Editors. 2026. FishBase. World Wide Web electronic publication.
- Seriously FishSeriously Fish. Heterandria formosa. Ficha de especie; consultada 2026-10.
Water: FishBase gives pH 7-8, 9-19 dH and 20-26 °C; Seriously Fish, pH 7-8, 5-20 dGH and 20-26 °C. The wider range is used: pH 7-8, 5-20 dGH and 20-26 °C. Size: 3.6 cm TL in FishBase and 20 mm SL in Seriously Fish, which gives males 2 cm; males are much smaller than females. Tank: Seriously Fish records no base and says a pair fits in 30 × 20 × 20 cm (12.5 l); the 40 × 20 × 25 cm base for a colony is the profile's own judgement. Salinity: FishBase lists it as freshwater and brackish and Seriously Fish says it also occurs in brackish water in some areas; the profile admits fresh or brackish. Name: «mosquito fish» is a name Seriously Fish records alongside «dwarf livebearer» and «dwarf top minnow», and it must not be confused with the mosquitofish (Gambusia).
Litres per fish, tank sizes and lifespan: the profile's own judgement. The sources consulted do not give them.
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