Originally Posted by redchigh
I only have two male platies (I think they're male anyway- I have no trouble sexing guppies, but can't figure out these platies.)
Alot of live bearers, platies included, can be sexed by looking for the gonopodium, which is an intromittent organ found at the anal fin of the male fish. This is from where the male gametes are injected. On males, it will appear as though the anal fin is more of a "stick," whereas, in females, the anal fin is typically more triangular in shape.
As for the original poster's dilemma, I've found that livebearers in general can get a little nippy with each other if the male/female ratio is off. Livebearers, for lack of a more civilized term, are hornballs, and will chase and nip at each other unless there is a sufficient outlet for their frustrations. Heh. Usually, if you have the gallons, I'd recommend adding more fish to diffuse the behavior. When setting up livebearer tanks in the past, I've used the ratio of one male per three to four females, and that's worked out pretty well for me. Experiences differ though, so make sure to get more opinions.