Powder blues are known to be quite aggressive. :-( How large is the tank? The answer may help us help you, if the tank dimensions are too small there might not be much you can do really, 3 tangs in a maybe too small tank and you add another large fish just spells trouble, even in my 6 foot long tank my tangs are relentless with any new additions and my tangs arent considered that that aggressive.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.