So what happens when you try to manage ich with an Acanthurus tang?
Let me speak from experience watching dozens suffer before I could catch them (always too late) to treat:
The parasite slowly increases its presence on the fish. Starts on the gills out of sight, then on to fins, then all over the head and perhaps other areas. They increase in numbers.
Simultaneously the gills become more and more damaged and the fish is increasingly less able to breathe and very slowly suffocates over weeks.
The fish can be fat in the belly but it's lateral line and bones will begin to show. The parasite literally sucks the nourishment and life out of the fish. A morbidly obese tang in the belly region will eventually appear emaciated throughout the rest of its body as it suffers more and more. They will scratch, breathe heavy, lose color, swim sporadically which fades to hiding and becoming less active. They will eventually stop eating, and the parasite will finally suck the remaining hint of life out of them.
If well fed and fat (lots of nori) the process can take 2-6 weeks before killing it off.
It is horrible to watch, I somehow justified it by claiming the fish was weak and would have died anyway. The above experiences largely negate that. It's awful to watch them suffer this way on a glim hope that I will magically start warding them off. With my success over the past few years with these fish I can't believe I used to have trouble before. This is so much easier and more rewarding (treating all fish and properly quarantining).
I've seen this process happen to hippo tangs that even disappeared for a few days only to emerge apparently ich free. I guess I expected this to happen with Acanthurus tangs. I assure you, it will not.
If you cannot afford to qt, or don't have the space, or don't want to deal with the trouble, then do the oceans a favor and either stick to only the hardiest species (some of which you will still kill) or perhaps leave the hobby.
It's our responsibility to these fish to keep them healthy.
