Is there anything new in the environment? Try to set your temp around 84 F, add a bit of salt, and don't forget to remove food if it refuses to eat. Please let us know how it's doing.
Danger signs: losing color, turning white, floating instead of swimming, listing.
Does it have a good bubble source? mine loved bubbles. How about any tankmates? I agree about the possibility of parasites too. Best of luck.