Tangs and Surgeons are more prone to ich than other species (so I've heard and read) I had the same problem as yours in the past, but now I have a regal tang (which i got over 8 months ago) and it is extremely healthy, active and thriving.
I think the trick is to pick a healthy 1 to begin with, watch it carefully for any signs of parasites over the first few days-weeks and treat it accordinglyl if disease/parasites show up. I did this with my regal tang, and so far so good.
Also watch the diet, feed plenty of mysis, krill, nori, add vitamins to the food and don't forget most importantly to feed garlic whenever u can.
Mayb u hv a parasite prob in ur DT that u r unaware of and it's only affecting ur tangs due to their suceptability to it. R they flashing or not eating or hiding for any length of time b4 they die? Mayb something went wrong during acclimitization? R u quarrantining them b4 they go into the DT? If not, u should really do so, it helped me alot when I got my Regal Tang.
Hope this helps, good luck.