My Pleco Is Sick Please Help?

Can you tell me what he has, please? I've had him for 4 years and he is very dear to me I don't want him to die. Symptoms are: Fins clamped open, hollow belly, lethargy, not eating, and he keeps hiding behind things in the tank. Nothing on the outside out of the ordinary that I can tell.. nothing like lost scales, sores, spots, or anything like that. He lives in a 55 gal tank with three goldfish and a small yellow belly slider turtle, lots of plants, I do regular water changes, and condition the water with Jungle Start Right and Aquarium Salt.