Least killifish

Overview

The least killifish (Heterandria formosa) is not a true killifish at all but a livebearer — and one of the smallest in the world. Females top out around 3.5 cm and males are barely 2 cm, making this North American native a true nano fish. It gives birth to a trickle of live young over days rather than a single big brood, and its gentle, peaceful nature and tolerance of cooler water make it a charming, low-key choice for a small planted tank.

Tank & water

A group thrives in a densely planted nano of at least 20 litres (5 gallons).

• Temperature: happy across 18–26 °C, so a small heater is optional in a warm room but useful for stability.
• Water: pH 7.0–8.0, neutral to hard — like most livebearers they prefer some hardness. Test with a kit.
• Gentle filtration: a soft sponge filter protects these tiny fish, plus regular water changes.
• Aquascape: dense planting, especially fine-leaved plants and mosses, gives cover for adults and fry.

Feeding

Least killifish are micro-omnivores with tiny mouths. Feed small live and frozen foods — baby brine shrimp, daphnia, micro-worms, cyclops — plus finely crushed or micro foods. They also graze biofilm and micro-organisms in a mature planted tank, which suits their constant, small feeding habit. Feed little and often, and make sure they aren’t out-competed by faster fish.

Tankmates

Best kept as a single-species group where their tiny size and calm behaviour can be appreciated. If you add tankmates, choose only the smallest, most peaceful nano fish and shrimp — nothing large enough to eat or bully a 2–3 cm fish. They breed readily and quietly, so a species tank will slowly populate itself. For more small-tank options, see the best nano fish for a small tank.

For other peaceful livebearers, compare the endler guppy or, at a larger size, the guppy.