To tell if a black molly fish is pregnant, look for swelling and a black pregnancy mark on its lower belly. Black molly fish are almost always pregnant, meaning the pregnancy mark is almost always there.
Black molly fish are short-tailed molly fish that give birth every 60 to 70 days. They usually give birth to between 10 and 60 baby fish, and the baby fish are usually about 1/2 inch long at birth.
Molly fish naturally eat their young, and if a fish breeder wants to discourage this phenomenon, he needs to move the pregnant molly fish to its own aquarium. Once the molly fish has given birth, it should be removed from the birthing aquarium and placed back into its old aquarium; this prevents the fish from eating its young. Alternatively, fish owners can densely plant their existing aquariums to give the young molly fish a place to hide from its mothers and other predators. This approach may help save some of the babies.
Black molly fish can breed with any other type of molly fish, including gold-dust mollies. To tell a male black molly fish from a female, look for a pointed anal fin and a large dorsal fin on the male.