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 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

Native to the Coastal Plain of the southeastern United States, from the Cape Fear River drainage in North Carolina south through Georgia and Florida to southern Louisiana; also introduced to some areas for mosquito control. One of the smallest freshwater fish species in North America. Males reach about 2 cm, females about 3.5 cm. Despite the common name, it's not a killifish; it belongs to Poeciliidae, the livebearer family. Heterandria is from Greek heteros ("different") + andros ("male"), referring to the extreme sexual dimorphism; formosa is Latin for "beautifully formed". Found in shallow, heavily vegetated margins of still or sluggish waters; also tolerates brackish conditions in some areas. The reproductive biology is unusual among livebearers: females practice superfetation, carrying multiple broods at different developmental stages simultaneously. Rather than dropping a brood all at once, they produce 1-3 fry over a period of 10-14 days. Reproduction is highly matrotrophic; unfertilized ova have very little yolk and nearly all energy for development is transferred from mother to embryo through a placenta-like structure, with embryo mass increasing by as much as 3,000% during development. Males lack courtship displays and force-mate. Whether females store sperm is debated; hobbyist sources indicate new mating is required for each pregnancy, unlike guppies and platies.

Breeding

Livebearer with superfetation. Instead of producing a batch of fry all at once like guppies, the female carries embryos at different stages of development and releases 1-3 fully formed fry every 1-3 days over an extended period. This means a healthy female produces fry almost continuously. Fry are tiny (about 5 mm) but immediately free-swimming and able to eat crushed flake and baby brine shrimp. In a species tank with dense vegetation, population growth is steady but not explosive because individual brood sizes are so small. Males mature at about 4-6 weeks. The superfetation strategy is unusual among aquarium fish and is one of the main reasons livebearer enthusiasts keep this species.

Common problems

The main challenge is their tiny size. Standard aquarium equipment (filter intakes, heater guards) can trap or injure them. Use sponge filters or pre-filter sponges on all intakes. The fish are hardy and disease-resistant once established. Ich is rare except in new arrivals; treat with gentle temperature elevation. Their drab coloring and small size mean they're overlooked in stores and rarely offered. Availability is the biggest practical problem; most keepers obtain them through livebearer clubs and specialist breeders rather than pet stores.

Bioload

Further reading

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