Have you checked the ammonia levels? Also, I know you mentioned the tank temp has always been where it is now, but if you lower the temp (gradually) I believe it would increase the oxygen levels in the tank. The only reason I say this is because when my Jag was bleeding, ammonia and low oxygen levels were the two main causes of bleeding gills(besides trauma... but you said there are no wounds or bites) that I could find on the web. Are you able to see the color of the inner gills? If they are graying in color that might be a sign of oxygen deprivation which causes gills to bleed. I would just try oxygenating the tank with a powerhead and lowering the temp as to try and ease its breathing so you can make it more comfortable in the meantime. Hope he pulls through and you are able to get some more suggestions/help on here. Good Luck.