That's a good kit, I think someone here told you about it
The tangs that you listed are very hard to keep even without treating them with anything. PB & PT have a very high mortality rate even when they come in great condition, which usually isn't the case. Anything can and will stress them out causing them to die, even just placing them in a tank. So its probably not the copper directly that is causing them to die but the fish and the condition that it came in.
Also you need to take into account where you're getting your fish from. If your LFS is getting them from a wholesaler then the fish has been treated already for a number of different parasites, and for the most part what you're seeing at the store are the hardier of the ones, as the weaker ones most likely died along the way to the LFS. Now if they're getting them directly from a diver, well that's a whole other thing. It's very likely, even probably, that the fish wasn't treated for anything or treated very little. The fish is probably already heavily stressed out and anything that stresses it out more could cause it to die.
It's also very important to slowly build up the copper over a number of days, this will give the fish time to adjust to it.