Least Killifish
Heterandria formosa
One of the world's smallest fish, a tiny hardy livebearer for nano tanks.
| Difficulty | Beginner |
| Min. Tank Size | 20 L |
| Temperature | 18.0–26.0 °C |
| pH Range | 7.0–8.0 |
| Max Size | 2.0 cm |
| Lifespan | 2-3 years |
| Diet | Omnivore |
| Temperament | Peaceful |
Overview
The Least Killifish (Heterandria formosa) is one of the world's smallest fish, a tiny hardy livebearer for nano tanks. It is a popular choice among Australian aquarists, reaching around 2 cm and living roughly 2-3 years when properly cared for.
This complete guide covers how to keep the Least Killifish successfully - tank size and setup, water parameters, the best foods (including recommended brands), suitable tank mates, breeding, sexing and the health issues to watch for. It is rated Beginner to care for.
Natural Habitat & Origin
This species comes from tropical freshwater habitats and does best when its tank reflects the conditions it evolved in.
Matching the temperature, water chemistry and cover a species evolved with is the simplest route to keeping it healthy and seeing its natural behaviour and colour.
Tank Size & Aquarium Setup
Provide a minimum of 20 litres for the Least Killifish. As a group-living species, swimming length and floor space matter more than height.
Live plants, driftwood and shaded retreats give security and show off the fish's colours against a natural backdrop.
Always add fish only to a fully cycled, mature tank with stable biological filtration. Match filtration generously to the fish's size and waste output.
Water Parameters
Keep the Least Killifish in stable water at 18-26 °C with a pH of 7.0-8.0. It adapts to a moderate range, so stability matters more than an exact figure.
Test regularly: ammonia and nitrite must read zero, and nitrate should be kept low with routine partial water changes. Always dechlorinate and temperature-match new water, because sudden swings cause far more illness than water that is stable but slightly imperfect.
Diet & Feeding
The Least Killifish is an easy-going omnivore that accepts a wide range of foods.
In Australia, good options include API Tropical Flakes or Pellets, Hikari Micro Pellets or Tropical, and Dymax frozen brine shrimp and bloodworm as treats. Feed small amounts once or twice a day (only what is cleared in a minute or two), vary the diet for the best colour and health, and avoid overfeeding, which is the leading cause of poor water quality.
Temperament & Tank Mates
The Least Killifish is peaceful and community-friendly. It mixes well with other calm species that share its temperature (18-26 °C) and pH (7.0-8.0) needs, are too large to be eaten and not large enough to eat it.
It is happiest in a group of its own kind and is shyer and less settled when kept alone.
Breeding
The Least Killifish is a livebearer - females give birth to free-swimming young with no special effort. Provide dense cover so some fry survive.
Even if you are not planning to breed it, recognising this behaviour helps you understand what you are seeing and respond well - for example by adding cover for fry or giving a guarding pair extra space.
How to Tell Males from Females
Differences between the sexes are subtle and most obvious in mature fish in breeding condition.
Common Health Problems
Like most aquarium species it can suffer from white spot (ich), fin rot and fungal or bacterial infections, almost always triggered by stress or poor water quality. Quarantine new arrivals for 2-4 weeks and watch daily for early signs such as loss of appetite, unusual hiding, clamped fins or laboured breathing - caught early, most issues are very treatable.
Prevention beats cure: keep water pristine, avoid overstocking and overfeeding, quarantine new arrivals, and act at the first sign of trouble.
Is the Least Killifish Right for You?
The Least Killifish suits beginners and experienced keepers alike, as long as you can provide at least 20 litres, water at 18-26 °C and pH 7.0-8.0, the right diet and a proper group of its own kind.
Meet those needs and the Least Killifish will reward you for around 2-3 years. Use our free aquarium calculators to plan your setup, and explore our other fish, plant and disease guides to build a thriving aquarium.
