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Care guide · Freshwater · fish

Least killifish

Heterandria formosa

Two female least killifish (Heterandria formosa)
Two female Heterandria formosa. Photo: Livebearerguy, CC BY-SA 4.0. Photo: Livebearerguy · Source · CC BY-SA 4.0Proportionally resized by existing media pipeline; no crop or colour changes.

The least killifish (Heterandria formosa) is one of the smallest livebearing toothcarps. Despite its English common name, it is not a true killifish. It comes from the southeastern United States and inhabits quiet, densely vegetated waters. Seriously Fish gives a 30 × 20 × 20 cm aquarium, about 12.5 litres, for a pair; this is a source-specific lower limit, not a general stocking guarantee. A larger, stable aquarium is easier to maintain well. Watch that each fish gets enough food and monitor water quality carefully.

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At a glance

Temperature
20–26 °C
pH
7.0–8.0
GH
5–20 °dGH
Aquarium length
At least 30 cm
Minimum aquarium volume
At least 12.5 L
Adult size
Up to 2 cm
Group size
At least 2 individuals
Temperament
Peaceful

About this species

Heterandria formosa Girard, 1859 belongs to the family Poeciliidae. Males are extremely small; females grow larger and deeper-bodied. The species is livebearing and is not a member of the true killifish family.

Natural habitat

The species occurs on the coastal plain of the southeastern United States, from North Carolina toward southern Louisiana. It inhabits still to slow-flowing, often freshwater and locally brackish habitats with dense vegetation. In an aquarium, provide gentle flow, cover and fine-leaved plants.

Aquarium and layout

Seriously Fish gives a minimum 30 × 20 cm base and 20 cm height (about 12.5 litres) for a pair. That minimum applies specifically to a pair; a larger group needs more space. Use a mature, gently filtered aquarium and keep a densely planted area out of strong current. Fine plants and moss provide cover for fry.

Water conditions

Seriously Fish recommends 20–26°C, pH 7.0–8.0 and GH 5–20 dGH for aquarium care. FishBase reports similar field values. Keep conditions stable; records from locally brackish waters do not mean aquarium salt is required. Change water chemistry gradually.

Behaviour and group size

This tiny fish is peaceful but vulnerable because of its size. Larger fish may bully or eat it. A species-focused aquarium or calm tankmates of similarly small size are safer than a typical community tank. The specialist source names a pair as the minimum; plants and low flow help the fish settle.

Feeding

The least killifish is an omnivore. Offer small live or frozen foods such as Artemia and Daphnia alongside complete dry food. Crush flakes very finely. Dried Daphnia is a supplement, not a complete staple; soften it and feed only tiny portions that are eaten promptly.

Breeding

It is a livebearing toothcarp. Females release small numbers of developed young over an extended period rather than in one large brood. Fry are tiny and need suitably fine food. Dense moss and plants provide cover. We do not give a fixed gestation period or guaranteed brood count.

Sources & review

This care guide draws on several specialist sources and was last checked on .

View the sources