By the looks of it, this goldfish may have swim bladder disease, which basically prevents it from swimming upright. So, the owner did what he thought would help the fish by suspending a piece of cork above the goldfish with a clothing tag not unlike what you would find on the back of your shirt.
Swim bladder disease is often associated with constipation and one of the treatments, as pointed out on Reddit, is to feed the fish tinned peas, either whole opt squashed. This is said to act as a laxative. Other foods to consider feeding a fish with swim bladder disease includes plants such as Elodea and Egeria and live foods such as Daphnia and brine shrimp. An easier solution is to feed your goldfish vegetable based foods instead of pellets or flakes.
John B. Virata has been keeping fish since he was 10 years old. He currently keeps an 80 gallon cichlid tank, a 20 gallon freshwater community tank and a 29 gallon BioCube with a Percula clown, a huge blue green chromis, and a firefish all in his kitchen, and a 55 gallon FOWLR tank with a pair of Ocellaris clowns, two blue green chromis, a six line wrasse, a peppermint shrimp, assorted algae and a few aiptasia anemones in his living room. Follow him on Twitter @johnvirata