Definition of mosquito fish
: either of two small, surface-feeding, freshwater fishes (Gambusia affinis and G. holbrooki of the family Poeciliidae) native to Central and North America that are live-bearers and are widely introduced outside their native range to control mosquitoes by feeding on their larvae York County has been populating water sources and backyard ponds with mosquito fish, a small guppy-like swimmer that devours mosquito larvae, for more than 25 years. — Amanda Williams
First Known Use of mosquito fish
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