Despite its name this is not a true killifish but a tiny North American livebearer — one of the smallest fish in the world. Hardy, peaceful, and perfect for nano and planted tanks, it gives live birth to a few fry over many days rather than all at once.
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Males ~2 cm (0.8 in); females ~3-3.5 cm (1.2-1.4 in) — one of the world's smallest fish
Lifespan
2–3 years
Social needs
group
Native region
Coastal southeastern United States (South Carolina to Louisiana, Florida)
Origin
New World
Climate
⛅ Subtropical
Water type
💧 Freshwater
Family
Poeciliidae
Genus
Heterandria
Part of the Livebearers
Egg-free breeders that give birth to free-swimming fry — guppies, mollies, platies, swordtails and their dwarf relatives. Hardy, prolific, and beginner-friendly favorites of the freshwater hobby.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Heavily planted nano
5 gal / 19 L
Heterandria formosa is one of the smallest livebearers (adults ~2 cm). A heavily planted 5-gal with sponge filter, neutral-to-hard water and 20–24 °C suits a small colony.
Photo coming soon
Recommended
Species colony tank
10 gal / 38 L species-only
More volume stabilises params and lets the colony breed steadily. Dense plants (moss, hornwort) shelter fry from being eaten. Avoid mixing with anything larger than a chili rasbora.
Photo coming soon
Ideal
Outdoor mini-pond
20 gal / 75 L planted tub
Seasonal outdoor planted tub or large aquarium with native plants, snails, and microfauna gives a self-sustaining biotope — natural foraging and full colour development.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Egg
Fish eggs are small, translucent spheres, often laid in clutches on plants, substrate, or in a nest — or carried/brooded by a parent in livebearing and mouth-brooding species. A dark eye spot and the curled embryo become visible inside as development progresses.
Photo coming soon
Fry
Newly hatched fry are tiny and semi-transparent, frequently still carrying a yolk sac that fuels them before they feed freely. They lack full fin structure and adult coloration, staying near cover until they can swim and forage on their own.
Photo coming soon
Juvenile
Juveniles look like miniature adults but with developing fins and muted or different markings; many species shift pattern and color as they mature. Growth is rapid at this stage given clean water and steady feeding.
Adult
Adults show the species' full size, finnage, and mature coloration, and are sexually mature. Many fish develop sex-specific differences in size, color, or fin shape, which can intensify during breeding.
Habitat & enclosure
A group thrives in as little as 20-40 L (5-10 gal). It is adaptable and even tolerates cooler temperatures: pH 7.0-8.0, temperature 18-26 C (65-79 F), moderately hard water, with gentle flow. It can be kept unheated in a temperate room and is a fine subtropical species.
Dense planting is key — fine-leaved and floating plants give security, grazing surfaces, and cover for fry to survive alongside parents. Moderate lighting supports plant growth.
Substrate
Fine gravel or sand works well; a darker substrate enhances color and confidence. Heavy planting matters more than the substrate itself for this fish.
Equipment & setup
A gentle sponge filter (which also protects tiny fry) and modest lighting are sufficient; a heater is optional given its tolerance of cooler water. No specialized equipment is needed — a small, well-planted, cycled tank is ideal.
Diet
Micro-omnivore. Feed finely crushed flake, micro-pellets, and small live or frozen foods (daphnia, baby brine shrimp, cyclops, microworms). Its tiny mouth needs small particles; it also grazes biofilm and microfauna in a mature, planted tank.
Behavior & temperament
Extremely peaceful and a little shy; best in a species tank or with other small, gentle nano fish (small rasboras, pygmy corydoras, shrimp). Keep a group of at least six with more females than males. Too small to combine with anything that might eat it, including larger livebearers.
Health
Very hardy and disease-resistant in clean, stable water. Its main vulnerabilities are being outcompeted or eaten by larger tankmates and sensitivity to swings in a small tank's parameters. Standard livebearer care prevents most issues; avoid overstocking the nano volume.
Tips, DIY & hacks
Because females give birth to just one or two fry per day over weeks (superfetation), heavily planted tanks let fry survive alongside adults without a breeding trap. A simple sponge filter keeps the tiny fish and fry from being sucked up. Excellent low-tech, unheated nano or patio-pond species in mild climates.
