Heterandria formosa

Common Names: Dwarf livebearer, dwarf top-minnow, least killifish, lesser killifish, mosquito fish, pygmy livebearer

Type Locality: Charleston, South Carolina and Palatka, Florida

Range: The Cape Fear River drainage in North Caolina to southern Louisiana; it has been introduced in some places as a mosquito control

Taxonomic Troubles: None. Despite the enormous revisions that have taken place among the poeciliids, this species still has its original taxonomic designation.

Size: maximum 3.6 cm (1.4 inches) female, 2 cm (0.8 in) male.

Preferred Water Chemistry: Subtropical freshwater, brackish; pH 7.0 to 8.0, soft to hard 160 to 350 ppm, 20 to 26 degrees Celsius (68 to 79 degrees Fahrenheit

Difficulty: Extremely hardy and adaptable. Good community fish.

Tank Setup: Planted tanks are perfect. Tankmates must be peaceful and tiny, too.

Feeding: Will usually take any aquarium fare. Small live invertebrates and algae are their natural (and favorite) foods.

Breeding: Easy. The fish is so small that rather than the monthly broods typical of poeciliids, the female practices superfetation, producing a couple of young on an almost daily basis. The fry are consequently large, much like the fry of other small poeciliids such as guppies. Thus, females are always gravid with young in all stages of development. Parents rarely eat the fry, and with plant cover, most will survive.

Fish Description

Marked sexual dimorphism--male slender and about half the size of females, with large gonopodium. Sexes colored alike--silvery gold background with faint dark mottling, a dark spot at the base of the dorsal fin, a thick horizontal stripe from operculum to tail, often showing faint vertical bars, especilaly on the rear half of the body.


This US-native fish makes an excellent aquarium subject. Despite its often being called a killifish, it is really the smallest livebearer and one of the smallest vertebrates in the world. Their peaceful nature makes them suitable tankmates for other diminuitive fish such as very small tetras or rasboras, or to provide variety in a killie community. While they may get lost (or eaten) in a regular community tank, these inch-or-less fish can be used to create nano community tanks--perfect for the popular destop aquaria.

A well-planted container with a few Heterandia and perhaps a shrimp or two would not necessarily require a heater or mechanical filtration. Even a sizeable colony can be maintained in a minimal setup--a 15-gallon with a thicket of Java moss can happily house a couple dozen.

Many aquarists summer these fish in outdoor tubs or ponds, and hobbyists in the Southeast coast can even collect their own (check local laws). Aquatic gardeners who like to raise exotic water blooms in outdoor containers can use H. formosa to prevent mosquitoes from successfully breeding while maintaining a minimal impact from the fish on their prize plants.

Not commonly found in retail stores, this delightful fish can be easily obtained from livebearer or native fish enthusiasts, and it ships well thanks to its small size. If you are looking for something tiny and terrific, try Heterandia formosa.