I had the exact same situation, but my Yellow Tang was in my tank for around 3 years. Definitely only get a Copperband that eats at the LFS. I printed a photo of a copperband and tapped it to the tank to see if the Tang would react to it and he didn't. After I added the Copperband, he didn't bother it at all. I did put the Copperband in my QT first to make sure he was eatting really good first. I had to hand feed him every night so he got enough food. I used live blackworms. He was the only Copperband that ate at the LFS out of 8. So, be patient. My first two only lived 6 months, but I wasn't feeding the blackworms. I had this guy for around a year before I sold him, he was picking on my clam. Actually, both him and the Yellow Tang went. Both were picking on the clam. HTH