"ich free" means the ich has just fallen off the fish to reproduce, and when the reproduction is done, it'll come back 100 times worse. Please read the ich stickies in the fish disease forum to understand the life cycle of marine ich (cryptocaryon irritans). herbtana is really just snake oil; lots of people tried it and most fail. there are no known "reef-safe" medications for ich. The only scientifically proven treatments are copper, hyposalinity, and tank transfer.
since you have the MI in a QT now, it's very to eradicate ich and don't waste time and money with "reef-safe" medications. Cupramine is very easy to use in a QT and very effective against ich. don't risk not curing the MI and introducing ich into your beautiful DT.