How do Live-bearing fish (Mollies, playtys and swordtails) have sex?
Look closely at the fish. The females have a full anal fin (underneath, in front of the tail). They keep it spread out most of the time. The males have a skimpy fin there, plus another piece that serves to deliver sperm. When the male is chasing the female fish (basically any of them not already pregnant), he will swim along side them, and swivel that skinny piece around towards the female. He aims for the base of that full fin. He will connect for just an instant — and then dart away. As the fry grow inside her, she develops a dark patch (gravid spot) that gets bigger as the babies grow. When her belly looks ‘square’ — that is, fat all the way up to her chin, she’s getting ready to have the fry. The females get tired of the males pestering them, and will sometimes chase the males away. Try to balance the tank with 3-4 females for each male, so that the girls get some rest.
