Black mollies are hardy little tropical fish that can tolerate water ranging from fresh to brackish, but thrive in freshwater best. The black molly is an excellent starter fish for those new to aquatic hobbies, especially if single sexes are kept. If you own both male and female black mollies, expect these prolific breeders to populate their tank with molly fry.
Black mollies are live-bearers, meaning that they do not spawn eggs like some other fish. The baby fish, also known as "fry," gestate inside the mother for a span of seven to eight weeks. When they emerge, fry will be fully formed -- all 40 to 100 of them. After being born, they should be removed from the tank, as the parents may attempt to eat some of the fry.
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As soon as they are born, black molly fry can eat adult flake food, although you may have to pulverize it some to make it easier for them to eat. It'll be important to separate the males from the females early on, as mollies reach sexual maturity at 8 weeks of age.
All Grown Up
By 4 months of age, black mollies are fully grown. And adult black molly can grow to be 5 inches in length, and if cared for properly, can live up to 5 years.
Mollies generally get along with most other tropical fish, and aren't prone to territorial aggression. There is the propensity for new molly owners to overfeed them, however, and they may steal food from other members of the tank. Feed black mollies only what they can eat in three minutes. If they are breeding, you should include live or frozen foods as a part of their diet. Bloodworms, brine shrimp and blackworms are all excellent choices.
Do You Want Fry With That?
If you decide to keep both sexes of black mollies in your tank, keep the females in your tank to a 2- or 3-to-1 ratio to the males. Males are naturally driven to keep up mating attempts. If there is only one female in the tank, she may become stressed and possibly die due to the constant demands from the males.
Fortunately, black mollies are very easy to sex; females are notoriously larger than their male counterparts. Male black mollies sport a stick-looking gonopodium, a modified anal fin that allows them to attach to the female mollies during mating.
There is little you will need to do to coax your mollies to bring about their next generation. As long as there are males and females in your aquarium nature will take its course, and you'll soon see a whole new cycle of baby black molly fry eagerly exploring their fascinating new world.