Least Killifish (Heterandria Formosa) is also known as the little killifish, dwarf topminnow, or midget livebearer. It is a fish species native to the southern United States. Coming from the Poeciliidae family, this fish is one of the smallest fish worldwide, and it is in fact the smallest fish that is native to North America. It is one of the few fish that is endemic to the North America. Although they are not the most colorful fish in the aquarium hobby, it is a unique fish that is very interesting to keep. They are calm and docile fish that can breed in your home aquarium.