Ich in tangs

[Proteus]

I don't think you can ever really be rid of it unless you haven't added anything to tank including frags or inverts.

When I had my 180 going I had a powder blue that would get a spot or two every couple months. Then it would disappear. I think it may have been a little stress. Considering to that none of the other fish would show sign. If your fish ate healthy and eat well and low stress there immune system will do its job