Do you know if the fish are wild caught (alot of Tangs are). What have you tried to feed? I would keep adding very small amounts of dry food every day or so untill they get hungry enough to eat. It might take as long as a week, but don't give up. Fish can go weeks without food, no problem. Once you have them on a quality dry food, you can start adding a variety of frozen to thier diet. If you spoil them on frozen right off the bat, it will be harder to get them to accept dry food later.