It is still safe to assume that the dt has ich and any new additions or stressed fish may have an ich outbreak while healthy, unstressed, and established fish will be unaffected. It is possible that the anthias may never show signs of ich. It is also possible that even after a successful qt treatment that the tangs come down with ich again as soon as they are put back in the dt. Not sure how much experience you have but I would suggest learning the life cycle of ich if you don't already. White spots on fish is the only time we can actually see it. And just because we can't see it on a fish, doesn't mean it's not in the tank in another stage.