Species Spotlight: Least Killifish (H. formosa)

The Least Killifish (Heterandria formosa) isn’t really a killifish at all. It’s North America’s smallest livebearer species. Found in slow moving freshwater creeks, bogs, ponds and tributaries along the southeastern United States, least killifish can be found feeding on daphnia, rotifers, grindal worms and other micro fauna such as mosquito and midge larvae. Peaceful, comical and active community tank fish, the least Killifish is ideal for a planted aquarium as small as one gallon. They make the perfect community fish for a nano sized freshwater tank and will even get along just fine with cherry shrimp and other neocaridina. They typically do not eat their young and happily co-exist with other small fish.

Their habitat and water quality needs are relatively undemanding. They prefer a slow current provided by a sponge filter with a pH from the slightly acidic to the moderately alkaline. Live plants are appreciated by Least Killifish, especially cover plants like salvinia and duckweed. Breeding is simple. As they are livebearers, you can encourage breeding conditions in the fish by feeding them plenty of high protein live foods. Being such a small species, the Least Killifish have developed a reproductive trick. Whereas larger livebearers such as guppies might have anywhere from five to thirty fry at once, the Least Killifish are so small that they don’t have the reproductive space for that. Their adaptation is that females rapidly develop and produce a single fry every other day or so. Instead of a lot of babies all at once every 30-40 days, the Least Killifish gives birth to their young more automatically, having the same number of babies in a shorter gestational time. Like other livebearers, females do store sperm from the males. The fry are relatively large and fully developed from birth. Feed them on vinegar eels, baby brine shrimp and microWORMS.