OK, so based on the added detail on the tangs, if I were you I'd start treating everything now and run fallow. You've caught this at the right time. Do the full treatment, then drain and restart the QT and add the clowns only after observing your display for a few months. Tangs (especially PBTs are the worst for Ich so you do need to treat, or they will die and leave an infestation that all the others (except maybe the foxface) will die by.