Fish all need some time to settle in to a new home. Give it a few days. What are you feeding currently? Tangs need a variety diet. Mine gets a varitey of frozen and high-quality flake. I feed frozen mysis, krill, some blood worms and cyclop-eeze. I feed spirulia flake and prime reef flake as well. I rotate what I feed and feed about three times a week. A couple times a week I soak he food in dietary suppliments like Selcon and Vita-Chem. I also add Garlic Extreme. Don't forget to hang dried sea weed strips in the tank for te tang to graze on. What size tank?
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian