Molly fish are good species for the beginning aquarium keeper. They are best kept with one male and one or more females. Unlike many fish species, the female does not deposit her eggs in a location, which the male then fertilizes. Instead, like mammals, the male fertilizes the female eggs inside her body with his gonopodium, a tube like organ. The babies, known as a fry, gestate inside until they are born live. To skip the molly fish mating ritual, you can buy pregnant females at the store. You can tell because females are slightly larger than males and they will have a big belly if pregnant.