I'm on my phone, so typing a bunch is difficult right now, but I just posted a response to someone else that has a Sailfin Tang with Ich. Check that out and search for Ich treatment on here. There are tons of posts (almost daily).
It's not the same as freshwater, so do not treat it the same way. There are basically two treatments, hyposallinity or copper, and both are done in a QT. Treating in your DT is not a good idea. Done correctly, you cure the problem. Using the "medication treatments" that most LFS
have "treats" the problem, but leaves it alive and able to reappear later.
Also...what fish do you still have and did you ever get that larger tank? i just saw an older post and it sounded like you had a lot of fish in a fairly small tank. If I were guessing, the Ich is on the Tang?