Try everything else before treating DT with copper. If you have done already, unfortunately you will eventually suffer the consequences. Try hypo-salinity and up the temperature (to just below what all your fish can handle).
Study ich...read, read, read!!! My tang is the only one that did survive after a nasty ich outbreak..which I found in the tank way too late. So even though your other fish look fine, tangs are quite hardy fish (and most often than not, the first one to show white spot on its body and if you get it in time, the one to survive).
Search what the ich parasite does in the tank. They can free swim for aaaaages before they find a host, eat away at your fish' flesh for a while then get off and reproduce. So that's why one day he may be covered then the next day he's only got a few spots, then the next day covered again. You need to kill the ich parasite when it's free swimming because it buries itself under the scales/flesh and the copper can't get to it. If they are covered in white spot (ich) do a quick fresh water dip on your fish the ich jump right off. They can't handle it.
That's why I picked hyposalinity and upped the heat and more bubbles in the tank (airstone or anything like that) as either way the high heat will absorb within the fish and hence effect the parasite under the flesh. Then it will unattached itself and die in the water with high salinity. The airstone is for the fish as changing the parameters makes the oxygen drop in the water.
Make sure you vacuum clean your substrate as well because most times that's where the parasite end up. If it doesn't get a host straight away.
Check these info https out:
I would get a UV
steriliser and put it in ASAP if you can afford it.
Also, start feeding your fish real fatty food to keep them healthy and to help them keep their appetite ( and in a way keep them far so the parasites don't "suck them dry").
Good luck and let us know what you ended up doing and give us regular updates so we can help you through it