Could be an internal parasite. I would isolate if hes doing okay and treat him with internal parasite guard or pepso food (becareful using it around inverts though!) Weight loss like that usually has to do with something in the gut.
I called and talked to a guy at my local fish store, and he suggested that I'm not doing enough maintenance on my tank. I do about a 25% water change each week. He asked if I ever vacuum the gravel and I said no I have never done that. Is that the problem?
My tank has a lot of plants (they are all doing well), and I thought they would take care of a lot of the excess ammonia and nitrate (I tested them and they are non-detectable).