topminnow

noun, plural (especially collectively) top·min·now, (especially referring to two or more kinds or species) top·min·nows.

any of several small, surface-swimming fishes of the egg-laying family Cyprinodontidae and the live-bearing family Poeciliidae, some of which are used in mosquito control.

noun plural -now or -nows

any of various small American freshwater cyprinodont fishes that are either viviparous (genera Heterandria, Gambusia, etc) or egg-laying (genus Fundulus)