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Encyclopedia / Livebearers / Least killifish
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Variations

1 kinds · tap one for a closer look

Least killifish

Heterandria formosa · Dwarf livebearer, Mosquito fish dwarf, Least livebearer

Peaceful and shy; keep with other small peaceful fish and plenty of floating plants.

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3.6 cmAdult size ✓
20–26 °CTemperature ✓
7–8pH ✓
12 LSmallest tank ✓
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Tank mates
Check your own mix →
Not recommended
  • Tinfoil barb eats animals up to about 5 cm. Least killifish only grows to 3.6 cm.
Not recommended
  • Giant gourami eats animals up to about 10 cm. Least killifish only grows to 3.6 cm.
Not recommended
  • Angelfish eats animals up to about 4 cm. Least killifish only grows to 3.6 cm.
With conditions
  • Only a narrow pH range suits both: exactly 7.
With conditions
  • Only a narrow pH range suits both: exactly 7.
Compatible
Compatible
Compatible
Compatible
Compatible

294 species in our list are a good match.

Facts checked against sources on 16 Oct 2026. ✓ means a source backs it; ? means no reliable source yet. See sources

Care and water

Hardness ✓5–20 dGH
Group size ?6 or more
Temperament ✓Peaceful
Swims at ?Top
Care level ✓Easy
Diet Omnivore
Lifespan ?Unknown
Size in shops 1.5–2.5 cm
Growth Medium

Behavior

Peaceful and shy; keep with other small peaceful fish and plenty of floating plants.

Food

Fine flake, micro pellets, baby brine shrimp, daphnia and microworms.

Tap a food to see what it is.

Male or female

Males are much smaller (about 2 cm) with a very large gonopodium; females reach about 3 cm.

Health

Common problems. Tap one to see the signs. This is a guide, not a diagnosis.

Breeding

Livebearer · Easy

Very easy and breeds readily with both sexes present. Females use superfetation (several broods developing at once, nourished through a placenta-like link), so fry are born a few at a time rather than in one brood. Adults usually leave the fry alone.

Pairing

Keep a group of 6 or more; they pair themselves.

Setup

Densely planted 20 L tank with floating plants, java moss and gentle filtration.

What starts it

Stable temperature and plenty of tiny live food.

Eggs

A few fry every day or two (up to about 40 over a 10-14 day period); gestation about 4 weeks. Fry take powdered food and Artemia nauplii at once.

Raising the fry

Fry take baby brine shrimp or powdered flake right away and need cover.

Common mistakes
  • Large tankmates eating the fry
  • Strong filter currents
  • Keeping alone
Full breeding guide, day-by-day plan and simulator →

Where it lives in the wild

RegionSouth-eastern USA
HabitatWeedy ponds, swamps and slow streams
Native rangeAbout 800 km across
See it on the world map

The story of the least killifish

The smallest known livebearing fish and one of the smallest fish in the world, native to the south-eastern US. Females carry fry at different stages at once (superfetation), so they give birth to a few young every day or two.

Charles Girard described this fish in 1859. Despite its name it is a livebearer, not a killifish, and one of the smallest fish in North America.

First described by Charles F. Girard in 1859. Named for 'Formosa' means beautiful.

  • Adult males are often only about 1.5 cm long.
  • Females give birth to just a few young at a time over many days.
  • Several broods develop inside the female at once.

Sources

14 facts checked with sources