Platies, wild type, are very peaceful fish John. On the other hand, many of the fish we see in a shop these days are called platies but their colors and other fancy features come from lots of cross breeding between the platy groups of fish and the swordtail group of fish. Both platies and swordtails are member of the Xiphophorus genus and they are often crossed to get that little something extra that neither one would have if not crossed. The trouble comes when a fish that looks like a platy is carrying lots of swordtail genes. Swordtails are not the peaceful fish that their cousins are. The most commonly used swordtails are from a X helleri ancestor, and X helleri males can be very aggressive when it comes to being with other helleri males. Some males in that group, I do have some wild type, do not even develop a gonopodium or a hint of a sword for over a year because it helps them avoid some of the aggression. It would be a bit unusual for a male to be aggressive with a female but not all that unusual between males.
