Instructions
Look at the body type of your mollies. Males have a slender, more streamlined shape. Females are more round and full-bodied.
Compare the individuals against each other. Males are smaller than females. The exception is for young females; they may be the same size as adult males until they are mature at just a few months old.
Look at the tails of your mollies. The males will have a gonopodium on their anal fins. This looks like a long, thin fin and is the male's sex organ.
Watch the behavior of your fish. Often the male is more aggressive and may be nipping or chasing other fish.
Look at the dorsal fin of your sailfin molly. In the sailfin species the dorsal fin is significantly larger in males.
How to Differentiate Male & Female Mollies
There are several species of Poecilia fish, most of which are commonly known as mollies, that make great additions to the home aquarium. They are small fish with each species having its own coloring and patterns. Selective breeding has created even more colors and patterns than nature provided. Mollies are easy to care for and easy to breed. A pair of mollies can populate a tank with dozens of babies quickly and often. Owners who want to breed mollies and those who don't are wise to learn how to tell the sexes apart. If you don't want them to breed, then you can choose to have only one sex in your tank. Since you can't tell by color, there are some key features such as size and sexual organs to look for to differentiate the male and female of most of the common species of mollies.