Been my experience with tangs (and I have & have had a few now) that not only is a proper acclimation necessary (highly advise a drip acclimation no matter the tang) but immediatly provide a fresh algae if possible, or else zoe soaked strips.
Tangs are sensitive to everything from the wrong look to the weakest of parasites.. I almost think they get 'sick' just because they know it'll get you to give them attention.
Highly recommend if you want to go with a regal or naso-esque etc etc to check out a 'product' called Tang Heaven from ipsf.com.
Basically the natural food for them, and if you have the dollars and space you can prop your own and feed them in doses.
Much better, imho
, then sheet algae or your average lfs
2 cents, take as ya will n good luck