Least Killifish (Heterandria Formosa)


Heterandria Formosa, also known as Least Killifish, Dwarf/Midget Livebearer, or Mosquitofish is a species from the Poeciliidae Family and it lives in vegetated slow-moving or standing freshwater or brackish waters of South Carolina, Georgia, Florida, Louisiana, and North America. These fish are not true killifish because they are viviparous (livebearer) while true killifish are oviparous (they lay eggs). They are benthopelagic fish which means they live and feed near the bottom of rivers as well as in midwaters or near the surface. H. Formosa’s life span is about 3 – 5 years.


Heterandria Formosa is known to be one of the smallest of all fish species and the smallest live-bearing fish. The average adult size is around 1.5 cm and the maximum size of an adult male is about 2 cm (0.8 inches) while the female is a bit bigger and around 3 cm (1.2 inches). They have a general olive color with a white stomach, a dark horizontal stripe through the center of the body, and a dark spot on the dorsal fin.


In females, a small black dot will appear on their anal fin when they reach sexual maturity. In males, the anal fins modify into a gonopodium (intromittent organ); an organ that is used to transfer the sperm and impregnate the females during mating. For males, it takes longer to get mature (40-50 days) than females and at a larger body width (10-14 mm).

Behavior and Compatibility


Heterandria Formosa is a shy and peaceful fish. They will do best when kept in colonies with their kind with at least twice as many females as males. Keeping the number of females higher than the males will prevent too much male interest in females and comfort them.

After a group of new Least Killifish is introduced in a tank, it may take a few weeks until they feel safe enough to spend more time in the open. For this reason, they need hiding places, larger plants densely planted (Amazon Sword plant, Windelov Java Ferns), and shaded areas.

Males will show some aggression when they are competing for the females. Provided hiding places will reduce injuries and fatalities caused by any possible wound infections.

Because the males are so small, they will be eaten by the commonly kept species. In other words, they will not do well with bigger species. They have the best compatibility with species that are non-predatory and similar in size. Pencil fish, Otocinclus, Leptolucania ommata, dwarf Corydoras, and freshwater shrimps are suitable tankmates. However, H. formosa can be aggressive towards shrimps (Caridina and Neocaridina) despite their docile character.

Water Parameters

Temperature: 10-32°C
pH: 6,8 – 8,0
Hardness: 5-20 dGH

Heterandria Formosa is an adaptable freshwater fish. It will do great at a wide range of water conditions of slightly acidic to mildly alkaline with a very wide range of temperature.

Proper Tank Conditions

It is considered to be a beginner-level fish and they don’t need extra-special conditions. Provide them with the water parameters they need along with plenty of hiding places, larger plants (Amazon Sword plant, Windelov Java Ferns, Java moss, and cryptocorynes), and shaded areas.

The minimum tank size required is 19 liters for a group of 3 fish (5 gallons). Heavily planted tanks with low water current are the best choice, but they can also be kept in a lightly brackish setup with more water current. Other than plants, the tank can be decorated with driftwoods, ceramic decorations, stones, and fine sand gravel at the bottom.

Also, the tank should have a lid without small holes because the least killifish tend to jump through them.


They have a carnivorous diet and they feed with anything that fits in their small mouths. They prefer small live foods such as daphnia, microworms and baby brine shrimps, mosquito larvae, and other small insect larvae. Since they are unfussy eaters, they can also be fed with frozen food, flakes, and pellets commercially available in the aquarium market. The dry or frozen food should be ground before feeding them to fit their mouths. Heterandria Formosa should be fed once or twice per day with small amounts to keep them well-conditioned and healthy.

The fry born relatively large and can take care of themselves, and can be fed with baby brine shrimp, microworms, and powdered foods.



Heterandria Formosa is a prolific and easy-to-breed species. Breeding is quite likely to occur in case they are provided with proper water conditions in an aquarium and both sexes are present.

The males do not have a special dance or display to court the females, they simply try to run to the females and mate with them. From time to time males may become too insistent and disturbing to females. For reducing the disturbance and stress on the females, it is significant to keep the female/male ratio over 2/1.

The females have a unique reproductive feature called superfetation, which means the female can have two or more ova of different ovulations successfully fertilized and the females will give birth to small numbers of fry daily rather than all at once. This feature enables to increase the genetic diversity since the females can mate with several males with different genes.

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