Least killifish

Quick facts

Adult size

2.5 cm

Lifespan

can live up to 3 years; short-lived for a fish; 2-3 years typical

Tank zone

all

Temperament

peaceful

Difficulty

beginner

Schooling

recommended 6+ (critical minimum 4, thrives at 10+)

Water parameters

Temperature

15–25°C

pH

7.0 to 8.5

Hardness

8 to 25 dGH

Tank requirements

Minimum volume

19 L

Minimum length

30 cm

Flow

low

Lighting

dim preferred

Substrate

any

Hiding spots

needed

Feeding

Diet: omnivore, feeds primarily at the all.

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.

Habitat

Native to freshwater and slightly brackish habitats across the southeastern United States, from North Carolina through Florida and west along the Gulf Coast to Louisiana. Found in ditches, ponds, marshes, slow streams, and the vegetated margins of lakes. The species (Heterandria formosa) was described by Girard in 1859. Females reach about 3.5 cm; males are significantly smaller at 1.5–2 cm, making the male one of the smallest vertebrates in North America. The body is olive to tan with a dark lateral stripe and sometimes a faint zigzag pattern. Not colorful, but the tiny size and interesting breeding behavior are the draw. The species practices superfetation: females carry multiple broods at different developmental stages simultaneously, releasing 1-3 fry every few days rather than producing a single large batch. This is unique among commonly kept livebearers. Hardy and adaptable to a wide range of conditions. Available from specialist livebearer breeders and occasionally in general pet stores.

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

Bioload coefficient: 0.3 (tiny livebearer; lowest-bioload livebearer in the hobby).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

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