Hyposalinity or copper treatments in a QT combined with leaving your DT fallow for about six weeks are the only proven cure. Other "treatments" that you see out there do not eliminate the parasite, therefor leaving it alive and ready to come back in the future.
Almost hate to ask, but what size tank do you have that Sailfin tang in? Anything less than a six foot tank is likely to stress the tang, which decreases immunity, which makes them more susceptible to Ich or other nasty things. Garlic treated food helps boost immunity, but is not a cure.
There are all sorts of links to articles on marine Ich. Read those so you understand how this parasite works. Read articles on treatment so you understand how to treat this properly.