The molly fish, otherwise just known as mollies, are a freshwater fish indigenous to Mexico and the Southern USA, are a hardy and easy to care for fish. From the Poeciliidae family, you can find mollies fish in freshwater rivers and estuaries and occasionally making an appearance in seawater. They are quite exotic-looking and will add variety and diversity to your tank. Since the molly fish is easy to care for, they are quite popular in the aquarium trade and they are closely linked to the guppy.