[Chin_Lee]

I really don't like to use copper on tangs. i had a powder blue that died rather quickly after the first day of copper treatment and the first day was not at the full strength treatment. but then again pb are rather sensitive and purples are just dogs (ime) so it might work. But do try garlic first and maybe a neon goby (cleaner goby) to eat that ick off the tang.
Its really bad practice but i've recently acquired a cleaner wrasse (despite their poor reputation to survive long-term in captivity so back off cleaner wrasse police) to assist with parasites and ick on newly bought more "expensive" fish. (ie Achilles tang). Its more like a sacrificial lamb to help the tang get through the first couple weeks/months and its kinda more like an investment than a novelty. Also I've been feeding the tank with newly hatch bbs that the cleaner wrasse goes crazy on so I'm hoping the cleaner will survive more long-term than normal.