well if it is that long they probably will be fine. fishes especially tangs are prone to white spots. all fishes are not 100% clean. if they are stressed out they will be prone to white spots. you will notice the fishes that have spent a long time in your tank will not be so easily infected even if a newly introdueced fish is badly infested with it. it works to a simple equation, stress= vulnerablilty to diseases. lower their stress level will allow them to be better protected from different diseases