more than likely, they're pregnant! :! I had a tankfull of black ones, and I ended up with several more! anyways, I think I found a way to not end up with multiplying mollies, and platties. switch the tank light out and replace it with a blacklight ( yes, a BLACKLIGHT!) and leave it on 24/7. doesn't seem to hurt the fish, and when you do want a light on them, turn a lamp on or something, but don't leave it on for more than a few hours. let the blacklight be their light, and you have to leave it on all the time, cuz I think the fish spawn in the dark. I know this s ounds crazy, but the blacklight works! good luck!