What kind of tangs are they? Are they they only fish in the tank? What else is in the tank? Corals? Inverts?
Generally reef safe meds arn't effective at ridding your tank of ich for good. They can and do help the fish's immune system fight off the ich so that it is not able to fully take over the fish's body. The ich is still present in your tank and is only one stress event away from taking lives. QT and going fallow are the only For Sure method of ridding your fish of ich. There are many methods of ridding your fish of ich in a QT though so that varies according to your preference.
I ask about inverts because running hypo in the DT is one In-tank method however it's difficult to execute properly and there are some hypo resistant strains of ich out there. You need a perfectly calibrated refractometer and an ATO to keep the salinity at exactly 1.009, If it creeps up even once over that number, the whole process and time clock starts over.