All Male Fish Behaviour

You just upset the pecking order by adding more fish, that's all. Now they are busy sorting out a new pecking order and Timmy has decided he doesn't want to be bottom of it - maybe even making a play for 'top dog'.  They'll sort it out in a day or two.

Then if you get more male platies, they'll do it all again 





A group of female bettas is worse than this. If this pecking order sorting gets a bit iffy, then if you do get more male platies you could try what female betta owners do.
When adding new fish, catch and bag all the existing fish of that species, leaving them floating in the tank, then re-arrange the decor. Once the new fish have been through the acclimation process, release both new and old at the same time. The theory is that re-arranging the decor makes the old fish think they are somewhere new and instead of just picking on the new fish they all have to start again from the beginning sorting out the pecking order.