To add:
Marine Velvet (amyloodinum) is sadly common with tangs too - almost as much as Ich is. But velvet is 10x more deadly, and 10x harder to identiy.... Ich typically resembles "salt" type "zits" on a fishes skin/surface. Velvet is much smaller - like flour sized pustules - but covering more of the body...looking like a fish with "bleached" spots on it...Many fishes (tangs too) always drop color/have bleached areas when sleeping/stressed and this is normal - making a positive ID harder... but with velvet - the fish will act just like it has ich...scraching, fast respiration, flashing, and bleached spots - but sadly, usually once it's verified to be present - it's already too late, as not only the one affected fish will die - likely any others sharing same waters will die too - and die within 24-36 hours. . . Ich usually takes a week to 2 to become mortal, but velvet can become fatal to all fish exposed within a day or two.
Worse - Hypo does NOTHING for velvet - as it can reproduce at SG as low as 1.003 nd as high as 1.036.. So anymore when receiving new fishes I automatically hypo them, then if ANY odd signs (scratching/fast breathing) occur during hypo (1.008 max) I don;t hesitate to treat the qtine tank with either copper meds or formalin. (which are the only 2 known treatments for velvet)