Least Killifish (Heterandria formosa)

From The Aquarium Wiki
Jump to: navigation, search

Least Killifish

Heterandria formosa-8072.jpg
Female Dwarf Livebearer

Heterandria formosa

19 Litres (5 US G.)

2-3 cm (0.8-1.2")


1 - 1.005


7.0 - 8.0

16 -24 °C (60.8-75.2°F)

5-20 °d

1:3 M:F

Pellet Foods
Flake Foods
Live Foods

3-5 years



This animal is available captive bred

Additional names

Least Killifish, Midget Livebearer, Mosquitofish


Originates from the southeastern United States from the Carolinas to Florida, where it tends to be found in areas of dense vegetation and low water current. They can sometimes be found in brackish water, like many livebearers.


Males are smaller than females and have a disproportionately large gonopodium. Like many livebearers, this fish is comically easy to breed and will multiply rapidly if in good condition.The fry are relatively large at birth and can eat powdered food, baby brine shrimp, and microworms at birth; they are seldom eaten by their parents. The female's unusual reproductive system (see below) results in 1-3 fry being born daily for several weeks instead of all the fry being born at once.

Tank compatibility[edit]

Peaceful with other fish, but their extremely small size and preference for slightly cooler water means that they must be mixed carefully. Other small North American natives, such as Leptolucania ommata and Elassoma sp., could be options. Like most livebearers, they should be kept in groups with at least twice as many females as males.
H. formosa is reportedly aggressive to shrimp, being said to gang up on shrimp, flip them over, and tear them apart! As a result, Caridina and Neocaridina shrimp may not be the best tank mates for these fish in spite of their otherwise docile dispositions.


Will take anything offered as long as it fits in its small mouth, but is chiefly a carnivore in the wild and should be fed as such. Flakes and freeze-dried foods are best ground up before feeding these fish, in view of their size, and frozen and live foods must be small, such as daphnia or BBS.

Feeding regime[edit]

The normal once or twice a day feeding regime will work well for these fish.

Environment specifics[edit]

Prefers a heavily planted tank with low current, but can also be kept in a lightly brackish setup as long as cover is provided. They do best at relatively cool temperatures: keeping them at temperatures above the mid 70's tends to result in strongly skewed sex ratios, though this is not a lasting problem if these temperatures are maintained for only part of the year. They can be kept in fairly large colonies in a tank as small as a 5 gallon as long as fish are regularly culled/sold.


A typical peaceful livebearer. Unlike other livebearers, the males do not have a courtship display, and instead they simply try to run up to the females to mate with them. In addition, females - unlike most other livebearers - practice 'superfeotation': they grow their fry in a fashion similar to an assembly line, with one or two fry being developed in sections at various levels of development: thus, fry tend to be dropped in small numbers daily rather than all at once.


An olive colored fish with a white stomach, a dark brown line through the middle of its body, and - in some specimens - a set of vertical brown stripes going through said horizontal line. Some fish have a small orange spot on their dorsal fin. Their small size and coloring make them very difficult to confuse with any other livebearer, and the presence of a gonopodium in males prevents confusion with some small killifish that inhabit the same areas.


External links[edit]