Mollies need a tank that provides three to five gallons of water per adult fish. They prefer a slightly alkaline pH of 7.5 to 8.2 and do best if you add a tablespoon of aquarium salt for every five gallons of water. For best results, change out 20 percent of the water in the tank daily. Keep the tank heated to between 78 and 82 degrees Fahrenheit.
To protect the fry (baby fish) after they are born, you need an aquarium net breeder, a breeding trap or a second tank in which to isolate the youngsters until they are big enough not to be eaten by adult fish. Alternately, you can place aquatic plants in your aquarium to provide hiding places for the fry.
You can buy black mollies at most pet shops. If the female you purchase is already pregnant, you don't need to buy a male, as a female can produce several broods of fry from just one insemination. If you decide to buy both male and female mollies, buy at least twice as many females as males.
Feed your pregnant black molly with a good-quality flake fish food. You can supplement the flake food with a few brine shrimp, freeze dried blood worms or live black worms daily.
When your pregnant molly looks very fat, either put her in a tank set apart for raising fry or in an aquarium breeding net or breeding trap. Once she has given birth, remove her from the isolation area and place her back in the main tank. If she gives birth in the main tank, don't panic. As long as you have provided plenty of hiding places for the fry, some are likely to survive.
Feed the babies powdered flake fish food. They are ready for release into the main aquarium population when they have doubled their length at birth.