Is this copper treatment in your main tank? If so, really major problem. If this is your qt tank, treatment has gone on too long. Keep reducing that copper to 0. You may have to observe them for a week or so and keep those parameters immaculate, with careful feeding and temperature monitoring. I would not put them into the main tank yet, because the copper may have caused issues that need to be watched closely.
Copper is a good treatment, but like many treatments for parasites, it is poisonous, and what you're betting on is the parasite dying before the fish does. Unlike arsenic, I believe copper will tend to excrete out, given good conditions. Your tang, if he's had the same treatment, may be scratching because his skin is uncomfortable: they don't produce as much slime coat as bottom fish. Also, tangs have a higher oxygen requirement than most fish, so you absolutely must make sure his water is oxygen-rich. Overfeeding in qt can really deplete oxygen.
In general, qt is for observation-only of a new fish. Do not treat unless illness manifests, for one thing, because copper only affects parasites: it will not cure diseases. So if something manifests, you diagnose first and decide on one of several treatments.
HTH.