Cryptonite, my male platy was such a bully to the females that I moved him to one of the goldy tanks. The goldies were small and he lived there for about a month, all going well, until he started fin nipping. He was such a terror, the little pearlies couldn't get out of his way fast enough. Sooo, I thought ... I'll fix your little red wagon and put him in with some big goldies -The Boisterous Boys!
All went well for a couple of weeks and he started it again. The Boisterous Boys are a rough mob and ten times his size but that didn't stop him, he would harrass them for fun it seemed. Now he is back in the community tank and I hope the presence of females will keep him preoccupied, if he starts harrassing again I don't know what to do, I wish I had a tank of commons - that would sort him out.