My 4" royal pleco has a sunken belly. I put in those sinking algae wafers but I don't think he eats them. A week a go I changed out his tank for a bigger one. and a cleaner one. No algae. So I don't know if he is starving or what.
Panaque nigrolineatus, as this fish is known scientifically, can be a very difficult fish to maintain. it should be provided with an aquarium of at least 45 gallons and plenty of caved and rocky areas for hiding. They generally lie around during the day, while feeding at night.
this fish is not an efficient algae eater and needs vegetables like zucchinni in addition to algae tabs. this fish also needs driftwood to rasp on.
I feed mine raw zucchini and he has multiple pieces of wood in his tank. He seems happy, and is growing. I have noticed that he won't eat cooked zucchini (so I sink mine with a fork) and that he prefers to eat the skin first. Good luck.
in this situation, it won't hurt to feed some bloodworms or blackworms. The fish is malnourished and that will help if it is still eating. Stop and switch to vegetarian food once it plumps up, though.