Female molly fish have a small anal fin below their bellies and males have a modified fin that looks like a short stick. Males also tend to be smaller than females.
Mollies are a live-bearing fish that make an interesting addition to the aquarium. Fish owners who want to breed mollies will need a male and female pair, who hopefully like one another. There are a few different varieties of mollies, but owners can always tell the gender of each molly by checking out their anatomy.
The male molly has a special fin called a gonopodium that is used for fertilizing the female molly. The gonopodium is a modified anal fin that looks like a narrow stick or straw. It may also feature a small section of normal fin tissue. The female molly has a half-moon-shaped anal fin in the same spot as the male sex organ. The sexual organs are fully identifiable at three months of age. This makes it easy to see at a glance if a fish is male or female.
There are a few other gender indicators that owners can look too. Females tend to be larger than males, but males will stay small if placed in a crowded tank. A pregnant female will display a large, tight belly that looks slightly squared as she gets closer to giving birth.