Least killifish

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Feeding

Micro pellets, crushed flake, frozen baby brine shrimp, frozen cyclops, frozen daphnia, and live micro-foods (vinegar eels, microworms, baby brine shrimp). The mouth is tiny; standard flake must be finely crushed. They feed at all levels but primarily in the midwater and at the surface. Biofilm grazing supplements the diet in mature tanks. Feed twice daily in small amounts.

Compatibility

• One of the smallest livebearers in the world. Females reach about 3.5 cm; males barely exceed 2 cm. Tankmate selection is limited to the gentlest nano species.
• Best in species-only tanks or with very small, peaceful companions: shrimp, snails, pygmy corys, and other micro fish. Even small community fish like neon tetras can intimidate them.
• Not actually a killifish despite the common name. Heterandria formosa is a poeciliid livebearer, related to guppies and mollies.
• Tolerates cooler water (15–28°C), making them suitable for unheated indoor tanks. Native to the southeastern US, so they're adapted to seasonal temperature variation.

Origin and habitat

Heterandria formosa, the least killifish, is one of the smallest freshwater fish in North America and, despite the name, is not a killifish at all but a livebearer in the family Poeciliidae, related to guppies and mollies. It is native to the coastal plain of the southeastern United States, from the Cape Fear drainage in North Carolina south through Georgia and Florida and west to Louisiana, and has been moved into some waters for mosquito control. Females reach only about 3.5 cm and males about 2 cm, a size gap reflected in the genus name, which means different male; formosa means beautifully formed. It lives in still and slow water among dense vegetation and tolerates some brackishness. Its reproduction is what sets it apart. Females practise superfetation, carrying several broods at once at different stages and dropping just one to a few fry at a time on a nearly daily basis rather than in a single batch. The species is also strongly matrotrophic: its eggs carry almost no yolk, and the developing embryos are nourished through a placenta-like connection, so nearly all of their growth comes from the mother during gestation.

Breeding

A livebearer that breeds through superfetation. Instead of releasing one large brood like a guppy, a female holds embryos at several stages and delivers a couple of fully formed young every few days over a long stretch, so a healthy female is producing fry almost continuously. The fry are large for such a tiny fish and free-swimming at once, taking crushed flake and baby brine shrimp straight away, and the parents mostly leave them alone, so in a planted species tank a colony builds steadily without the explosive surges of other livebearers. Output rises with more food and lower crowding. Males mature in about a month.

Common problems

Their extreme smallness is the main practical issue: standard filter intakes and equipment can trap or hurt them, so sponge filters or pre-filter sponges are needed. They are otherwise hardy and disease-resistant once settled, with ich only a real risk on new arrivals, treatable with a gentle temperature rise. Despite their size they are not safe with dwarf shrimp, since they will harass and kill them. The bigger obstacle for keepers is simply finding them: they are plain-looking and rarely stocked in shops, so most come from livebearer clubs and specialist breeders.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size