first off, sorry about your loss. um im not big on royals but i no your not supposed to treat the water with medications, or do so in small amounts. how much you use? The medications become concentrated in the algae and bogwood, and when the fish eats these, it often is poisoned to death. These fish are also subject to intestinal disease if not fed the proper diet. They are very sensitive to changes in water chemistry, has there been any change? Recently imported specimen often suffer from malnutrition and as a result are hollow-bellied. Thus it is important to offer them high-fiber foods in order for them to recover. Algae; fruit; vegetables; peas, spinach, lettuce; live; crustaceans, insect larvae. Panaque species must have a large amount of "green foods" in their diet or they may develop intestinal disorders.
a few other questions- what were you feeding it. how long was it "ill". what were you're water peramiters? did it have many places to hide, or any more agressive and faster feeding tankmates? was it feeding correctly? and was it just the skin of it that made it look ill, or were there any other probs like sunken belly etc.