If you think your goldfish may have eaten a dead plecostomus, it is important to monitor them closely for any signs of illness, such as loss of appetite, lethargy, or difficulty swimming. If you notice any of these signs, it is best to take the goldfish to a veterinarian for evaluation and treatment.
In the meantime, you should remove any remaining parts of the dead plecostomus from the tank and clean the water thoroughly to remove any bacteria or toxins. You should also keep an eye on the water quality and make sure that it is within the ideal range for goldfish.