Hard to tell from the pic but could definitely be HLLE. I had issues in my 120 with HLLE that affected a tang, cherub angel, and then a regal angel. My best guess was stray voltage that had been in the system for quite some time that I didn't realize was there. IME there is not much you can do, other than make sure your water quality is very good, consider removing carbon, find a source of stray voltage if it exists, and feed a more varied diet with dietary supplements (i.e. selcon or something similar). That still might not be enough, as HLLE is a bit of a mystery. You might be able to stop the progression, but a full reversal of symptoms is likely going to prove difficult.