The southern platyfish, common platy, or moonfish (Xiphophorus maculatus) is a species of freshwater fish in family Poeciliidae of order Cyprinodontiformes.

Wild varieties are drab in coloration, lacking the distinctive dark lateral line common to many Xiphophorus species.

Breeders have developed a multitude of color varieties (e.g. orange, red, yellow, red/black, and black/white) which are common aquarium fish for hobbyists.

Often the distributions of the damselflies and introduced fishes were found to be mutually exclusive, probably resulting from competition for limited insect food.

Often young are eaten by the adults or other inhabitants of a communal aquarium but given plants and gravel to hide in, some will probably survive as these are hardy fish.

(However, this basic assessment does not take into consideration the undeniable fact that the majority of fishes sold as 'platies' do share the same stout form as the wild X. maculatus, whilst aquarium 'hybrid' swordtail strains likewise share the longer, larger body-form of the wild X. hellerii, so caudal extension is not the sole differentiator and it is clear that decades of selective breeding have reinforced, within each captive strain, the appropriate or typical – and therefore desired – body shape of the name types.)

A common statement in the trade is that it is harder to stop them breeding than to make them do so, with ‘surprise’ fry appearing in community tanks regularly.