This is correct, chances are the tang's from Hawai'i. However, this doesn't rule out the possibility of parasitic disease, and as far as I know, not all yellows are from Hawai'i, as they have collections limits, again, to the best of my knowledge. This would imply that a gap might be left between demand and supply, yes?
Garry is spot on about how fishes, especially poop machines like tangs are handled pre-ship. Stressor #2. (#1 is capture, whether or not with cyanide.) Also, some collectors and shippers are good at what they do, some are not so good. Stressor #3. Then we have the issue of the airline handlers at whose mercy are the fish (as well as our poor luggage--I watched once and was sickened, and it was inanimate luggage!). Stressor #4.
Alright, mooner, here's the thing: if you've got a non-Hawai'ian fish, it very well may have been exposed to cyanide, which means that even if you could get it to take food, its gut-lining is unable to absorb the available nutrients. I have NO idea if there's been research equivalent to, say, horses, in regards to uptake percentages of nutrients and all that, but since the fish isn't eating at all it's a moot point.
So, the fish won't eat, you can't exactly pickle him in nutrients as that doesn't work. Have you tried LIVE foods of any kind? If not, try live brine, much as I hate them, live mysids, even live bloodworms--anything to kick in that feeding response. It's my opinion that two things must be done at this point; get him EATING, anything, and I think it may be time to begin an antibiotic regimen. My first choice is Spectrogram. My LAST choice (mostly due to communiques with Mr. Calfo) is Melafix. This, along with the freshwater dips, and I will say again that I agree with Garry's assessment, are the animal's best chance.
If it's an internal parasite, hopefully triggering feeding and the antibiotics will give the animal just enough boost that it can tolerate an anti-parasitic regimen.