Eastern mosquitofish

The eastern mosquitofish, eastern gambusia, or mosquito fish ("Gambusia holbrooki") is a fish that belongs to the Poeciliidae family. It is from the eastern United States but human beings brought it to many other parts of the world, such as Hawaii, Puerto Rico, and Australia, where it has caused problems as an invasive species.
The adult male eastern mosquitofish grows to 3.5 cm and the female to 6.0 cm. It is green or brown on top and blue, white or silver on the bottom. It has large eyes and one dorsal fin and a round tail fin.
This fish likes to live in fresh water but can live in water that has some salt. It can survive cold and warm temperatures. It eats insects and their larvae.
People brought the eastern mosquitofish to Hawaii in the 1910s and to Australia in the 1920s and set it free there. They thought it would eat mosquito larvae so there would be fewer mosquitoes. But the eastern mosquitofish did not eat more mosquitoes than the fish that were already in Australia did. The eastern mosquitofish outcompetes other fish and bites their tails or even eats them. It can kill larger fish by damaging their fins. In the western United States, the eastern mosquitofish killed or outcompeted many other fish that were better at controlling mosquitoes than it is. After people put eastern mosquitofish in a body of water, there may be an algal bloom because the eastern mosquitofish eats or kills all the fish that usually eat the algae.
This fish likes to eat frog eggs and tadpoles. It is one reason why the green and golden bell frog is endangered in Australia and the Chiricahua leopard frog in Arizona. 
The fish has 50-300 eggs at a time. The eggs stay inside the female's body until they hatch. It takes the new fish two months to become adults. Eastern mosquitofish can do this several times each year.
The western mosquitofish "G. affinis" and eastern mosquitofish "G. holbrooki" used to both be called "G. affinis" because people thought they were the same species. They are not. According to the IUCN Global Invasive Species Database, most records in Australia that say "G. affinis" really refer to "G. holbrooki".