A couple of things have happened. Your P wants to be dominant. Your tank is setup so he/she can setup a territory. So in order to fix that you need to rearrange your tank by taking some of your decors out or add more P's. When you rearrange the tank make he/she cannot make an association with an object it mark its territory. . Reds do really well when they are feeling a little tight. They seem not willing to fight each other in a crowed tank. Also by adding another red of equal size the dominant piranha will eventually give up its territory because there are too many challengers.
SMTT
SMTT