I have a purple tang in my 55 and it is a very nice fish.. I will be upgrading to a 90 soon for the tang police learking about..
My buddy has had a PT in his 55 for years with no issues and it is a fat and happy fish. I think the key to keeping them happy is making sure they have enough swimming room. A tank packed full of LR
cuts down on room for the fish. Again just one of my fav fish that I have. I keep the actinic lights on later ever night so I can check him out.. He has great color under the actinics.