You'll find that there is a stage where it will drop off the fish. If you're observant enough (and have enough space), that's when you want to remove the fish from one QT to a clean QT before they get to reinfect the fish. Obviously that's a bit of a handful when you have a few fish to deal with, as the lifecycle of the ich might vary enough between the infected to make it awkward.
Failing all that, you can just accept that ich is a common problem, and as @
riley mentioned, good husbandry and reduction of stressors to the fish will aide greatly in ich "going away" on its own. It seems to appear only when fish are stressed, and often seems to vanish when the stress is gone. Obviously it's never really gone, it's just not a serious problem until the stress returns and the fish wind up covered in spots and start dying. I think you'll find that a lot of tanks a like that - especially tanks with a lot of tangs. The problem just isn't evident until something causes stress.
Actually getting rid of it properly would be better, but for a lot of people it's just impossible to QT that many fish. A lot of conjecture exists on the subject, and there are a lot of ways of dealing with it, and yet the end results are questionable; ich can "go away" on its own or you can treat it away - the point that's ignored in determining what works is whether or not either makes any difference if the same stressor returns to the system (ie. if it appeared out of no where, it was already present in the system for however long.. which may be a long time).