I was just hoping for " unhealthy chubby" rather than "suffering disease" but thank you for the insight and confirming nothing I can do.My thought is that the RG may be suffering from edema. The general thickening of all tissues is not how fish store fat. More typically, the fat is deposited in the belly and it displaces the vital organs - so the fish may never "look" fat.
Edema can be caused by kidney or liver disfunction. It causes fluid buildup in the tissues. It typically isn't treatable. Ascites is related, but it is characterized by loose fluid in the belly area.