Least Killifish
Least Killifish is also known as Heterandria formosa. It is outstanding in many ways. One of its distinctions is that it is undoubtedly the smallest of all livebearers. The mature male is only half an inch long and is distinguished from the female by its telltale anal fin; the female may be an eighth to a quarter of an inch longer than the male. This fish has been given the name "mosquito fish," perhaps because of its size, but mainly because of its importance as one of the three types (along with the guppy and the gambusia) of fish that are used for mosquito-larvae destruction.
The life cycle of the mosquito is an interesting one. The larvae and pupae of the mosquito are aquatic and wriggle around in the water. After a certain length of time, the larvae change to pupae that metamorphose and fly away as full-grown mosquitoes. Once the insects have left the water, there is no practical means of disposing of them in great numbers, but while they are in the aquatic stage there are several techniques. One is to pour oil on the water so that when they come to the surface to breathe atmospheric oxygen, the oil slick will suffocate them. This method has its drawbacks, as it presents a fire hazard, frequently destroys other wildlife besides mosquitoes, is unsightly, and also must be done several times a season. An alternative method is spraying with chemicals, but this method also has its disadvantages. The third method (there undoubtedly are more, but they are all variations of these three) is to infest the water with fish that will eat the young mosquitoes. This need be done only once, as the fish, instead of becoming less effective like the chemical measures, multiply and become more effective. The fish can eat their weight in mosquito larvae daily. Many aquarists actually bring mosquito pupae into their homes and feed them to the fish. But the problem of how to become rid of the adult mosquitoes once they have transformed is another story.
The breeding habits of H. formosa are quite remarkable. The female drops a few fish every day and this feat may go on for a few weeks at a time. If there are quite a few females in the tank, you will probably find two or three new offspring every day. Sufficient vegetation should be provided in which the young can hide.
The optimum temperature for the breeding of these fish is about 70°F., and, as is true of the majority of fish, they fare better when kept only with members of their own family.
