Heterandria

The least killifish, Heterandria formosa, is a tiny livebearer from the United States, found in heavily vegetated margins of still or sluggish water. The smallest known livebearing fish, it is peaceful but not a community fish in the general sense, as the males reach only 2 cm and many commonly kept species will eat them. It is sometimes confused with the much more aggressive mosquito fish of the genus Gambusia.