Contrary to popular belief, sex of mollies you can define the shape of the tail and anal fin. This fin is on the ventral side of the fish closest to the tail fin near the anus. Females mollies anal fin is triangular in shape, males have the same but it is modified in copulatory organ called gonopodia, and has the shape of a tube. Female mollies can only spread and stack your fin, while the male can move them in all directions. This feature will help you to determine the sex of all live-bearing fish (which include mollies).
Fry choosing, remember that newly born fish of both sexes the anal fin is straightened. So do not rush with the purchase, wait for a few weeks.
Compare the sizes of males and females. Have Mollienesia sphenops females significantly larger than males and can reach twelve inches in length, while males rarely exceed eight inches. Moreover, a stronger and more successful the manufacturer will be small and nimble fish. Have Mollienesia velifera, on the contrary, the male is larger than females.
If you get adult fish, Mollienesia velifera males can be easily distinguished by the huge dorsal fin, through which this kind of mollies and got its name "sailing". It typically grows a half years. Females of this species the dorsal fin is of normal size.