The green swordtail ('Xiphophorus hellerii') is a species of freshwater/brackish fish in family 'Poeciliidae' of order 'Cyprinodontiformes'. A live-bearer, it is closely related to the southern platyfish or 'platy' ('X. maculatus') and can crossbreed with it. It is native to an area of North and Central America stretching from Veracruz, Mexico, to northwestern Honduras.

The male green swordtail grows to a maximum overall length of 14 centimetres (5.5 in) and the female to 16 centimetres (6.3 in). The name 'swordtail' is derived from the elongated lower lobe of the male's caudal fin (tailfin). Sexual dimorphism is moderate, with the female being larger than the male, but lacking the 'sword'. The wild form is olive green in color, with a red or brown lateral stripe and speckles on the dorsal and, sometimes, caudal fins. The male's 'sword' is yellow, edged in black below. Captive breeding has produced many color varieties, including black, red, and many patterns thereof, for the aquarium hobby.

One of the most popular tropical aquarium fish, the green swordtail has been bred into various hybrid forms for the aquarium hobby due to its hardiness and suitability for community tanks.

More Info: en.wikipedia.org