Hi there, first you need to understand that a copperband is no guarantee that they will eat aptaisia. In my time i have seen not many doing that. Then second question, what do you feed or fed those that did not survive ? They also eat some live foods and not ALL will take to frozen mysis or foods we offer. I don't think the tangs has any impact on the health of a copperband. How big is the tank ? I have successfully had tangs and copperbands together but was in a 500 l plus tank. But i used to breed brine shrimp specially for my copperband.
Looking at what you wrote, i am of the opinion that the reason the copperband stopped eating was because the copepod population was depleted. Not related to the tangs at all unless there is visible bullying going on.