FWIW - I tried 3 or 4 products when my tang had a few white ich spots, and a lot of black. None of the other fish every had it. I think they were called Ich-attach, Rid-Ich, and a couple others. I was using garlic additive at the time. The spots would come back after 4-5 days.
Nothing worked except a couple FW dips to get the current pararsites to drop off, and adding a UV. I did isolate him for about a week, and used coppersafe.
He DID develop a mild case of HLLE after the FW dips and coppersafe, so be careful, but Tangs are susceptible to that anyway, it could be his diet (its since improved).
UV isn't a complete solution, but it sure helps.
I'm over-doing it to the point of sillyness. I've got a (albeit cheap) 34W Jebo UV running on my main return line, so it catches everything sooner or later, and I'm running it at a low GPH, under 200. That should about obliterate anything passing through (even pods, which I'm working on a solution for).
I just took the UV apart to check if it needed cleaning, and the Jebo was built a lot better than I thought it would be. A nice quarts sleeve around the lamp, and the sleeve was perfectly clean after 2-3 months of use.
I haven't seen any spots (black or white) on my Tang for over 2 months, and they're the most susceptible to ich.
Again, FWIW. Some people just don't have the option to quarantine their entire community.