The Medaka (Gambusia affinis) also known as the mosquito fish is an aggressive introduced fish. It got its name, mosquito fish, because it was brought to Hawaii to help with mosquito control. It is silvery olive in color and the head is darker than the belly. The females have black dots on their back fins.

The Medaka is found mainly in vegetated ponds, lakes, rivers, springs, and ditches. The prefer shallow, slow moving water.

Besides mosquitos, it likes to eat zoo plankton, algae and detritus. Because the mosquito fish is known to be aggressive in some habitats, it is known to have been responsible for the reduction of certain native species.