The Southern Platyfish, Common Platy or Moonfish (Xiphophorus maculatus) is a species of freshwater fish in family Poecilidae of order Cyprinodontiformes. A live-bearer, it is closely related to the green swordtail (X. helleri) and can interbreed with it. It is native to an area of North and Central America stretching from Veracruz, Mexico, to northern Belize.
The southern platyfish grows to a maximum overall length of 6.0 cm (2.4 in). Sexual dimorphism is slight, the male's caudal fin being more pointed. The anal fin of the male fish has evolved into a gonopodium, a stick shaped organ used for reproduction. The female southern platyfish's anal fin is fan shaped.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 which are common aquarium fish for hobbyists.
The southern platyfish is commonly known simply as the platy (pl. platys or platies), from the fish's original generic name, Platypoecilus.
In captivity, they reach maturity in 3–4 months and breed readily, the females giving birth to about 40–50 young at a time.
The fish commonly sold in pet shops is not a pure strain of X. maculatus, but is a hybrid between X. hellerii and X. maculatus. In general if the male has a sword shaped tail they are called swordtails. Otherwise they are labeled platy. Color and fin shape vary wildly in the aquarium trade.
A common statement in the trade is that it's harder to stop them breeding than to make them do so, with "surprise" fry appearing in community tanks regularly.