I don't know why every book and website has platies listed as peaceful. I've always considered them semi-aggressive. Its not because of fighting, but because they constantly pick at the slime coat of bigger fish. I've got my three new platies in with my shubunkins right now and they are picking away at them, which is the reason I have them with my biggest and fastest goldies. The poor goldies are going crazy trying to get away from these little parasites. They might go into the new tank sooner than later as I don't want my shubbies stressed.