Gambusia holbrooki was deliberately introduced into Australia in 1925 because it had a reputation for mosquito control. It turned out to be no more effective at eliminating mosquitos than our native insect-eating fish.

It has had a significant impact on our native fish, invertebrates and frogs. It is an aggressive species - chasing and nipping at other fish in competition for food and resources. This nipping causes injuries which may become infected and potentially lead to the death of the other fish. Along with a variety of insects, Mosquitofish eat the the eggs, larvae and juveniles of native fish. They also eat frog eggs, tadpoles and adult frogs. As a result, the Mosquitofish has been associated with the decline in numbers of at least nine Australian fish species and over ten frog species.

Mosquitofish prefer warm, gently flowing or still waters, but they are extremely tolerant of a wide range of environmental conditions. They can survive in water temperatures from near freezing up to 38ºC, water with very low dissolved oxygen levels, and in a salinity range of freshwater to fully saltwater.

The species has a dorsally flattened head, large eyes and an upturned mouth. Female Mosquito fish average a length of about 6 cm. Males are much smaller, almost half their size at 3.5 cm. They may be pale silvery-grey all over or have an olive-green to brown back, blueish-grey sides and a silvery-white belly. Adult females often have a black blotch near the abdomen and males have a distinctively long anal fin.

Mosquitofish have a high reproductive rate. Breeding occurs in the warmer months of the year, with up to nine broods over that time. Each bood produces an average of 50 young, although up to 300 has been recorded. Mosquitofish are live bearers. Females can store sperm and self-fertilise in the next season, so if they can't find a mate they can still reproduce. The fertilised eggs develop for three to four weeks before the tiny larvae are born. The fish are mature and ready to breed in under two months.