Gambusia

The mosquitofish, Gambusia affinis (Baird & Girard, 1853), is a species of freshwater fish. It is a member of the family Poeciliidae of order Cyprinodontiformes.
These fish are native to the watershed of the Gulf of Mexico, where it has long been known that they feed readily on the aquatic larval and pupal stages of mosquitoes.

They are remarkably hardy, surviving in waters of very low oxygen saturations, high salinities (including twice that of seawater), and high temperatures; they can even survive in waters up to 42°C for short periods.

For these reasons, this species may now be the most widespread freshwater fish in the world, having been introduced as a biocontrol to tropical and temperate countries in both hemispheres, and then spreading further both naturally and through even further introductions.