72 hours is more then enough time to treat any fish with copper, any more may cause kidney failure.
Clown tangs are one of the most active in the tang family, they swim constantly. Think of them as a marathon runner, they need to feed an active body to be healthy. The first hurdle you have is to find out what he will eat, once you establish that you can start to build into higher protein foods to keep him healthy and alive. With any luck you will get him to eat calurpa, once he eats this his natural food source, get him eating mysis with spectrum pellets mixed in, once he starts eating this you should be home free...
Good luck! Keep in mind athletes need to eat alot.. ;)