fin erosion is usually fin-rot. Ask and post in the Fish Disease forum. Treatable in quarantine, with antibiotics. I'm also seeing a little ruddiness on the hind end, which is also characteristic of a bacterial infection [fin-rot is]. If he is getting picked on, [unusual for anyone to approach a tang's tail] that could start it, but it may just have come via a reduced condition and a little ding. It's like an infected sore, and needs treatment.