He might be hiding. If he is, when he comes out, you can set him up in a hospital tank, and use Melafix. You can get it just about anywhere, along with stress coat to regrow his fins. It works within 2 weeks. When I bought my YTBD (gone now was too aggressive) he had almost no tail, started using those in combination, and they were back to normal in a week an a half. And to keep their fins in order I dose SC and Melafix every week or two, just to help with the fins and scales. For the record, Damsels are extreeeeemly aggressive! Sure they're cute n stuff, but they pick on EVERYTHING. When I added my feather duster, and Star Polyups he tormented them for days, wouldnt let them come out at all. Finally he got used to them, but then when I added the peppermints he got super aggressive and wouldnt leave them alone, finally I got rid of him. Now I have an Orchid dotty in there. Doing great, but he killed one of my shrimp, but seems to have befriended the other...