Heterandria formosa

Heterandria formosa is the scientific name used when the species is sold under its binomial rather than its common name of Least Killifish. This tiny poeciliid is endemic to the southeastern USA and is celebrated for its miniature size — males are among the smallest vertebrates at 1.5–2 cm — and its extraordinary superfetation reproductive strategy.

The bold black lateral stripe running from snout to tail base is the most conspicuous marking on both sexes and is visible at even the smallest sizes. Females are slightly larger and conspicuously rounder when gravid. The species is an obligate inhabitant of densely vegetated, slow or still water with abundant surface coverage.

This entry allows the species to be found under its scientific name by customers who know it from specialist literature or killifish society sources.

Water: 20–26 °C, pH 6.5–8.0, soft to moderately hard; wide tolerance makes it forgiving. Tank: 20–40 L; densely planted with floating cover; sponge filter only; no powerheads. Feeding: Micro worms, baby brine shrimp nauplii, infusoria, and nano particle flake. Breeding: Superfetation; 1–2 well-developed fry produced daily over weeks from a single mating. Compatibility: Nano species tank; only suitable companions are Boraras, Danio erythromicron, or similar micro species.