The Purple Tang is notorious for being pretty mean and most people get away with multiples because they have a large tank. You can also use the school mentality which is stock a lot of them so none of them become overly dominant. That said, a 90g is already a small tank for Purple Tang and in such a confined space it wouldn't be a huge suprise if a Purple Tang harrassed another zebrasoma to death. My Purple Tang in my 120g went after my hand any time it was in the shape of a zebrasoma tang. He was put into Sals huge tank with a host of other tangs and calmed right down. Had I put a yellow in with him, he would have killed it for sure. Might have taken a few months but he would have eventually bullied it to death.
