air bladder issue most likely.. and I wouldn't doubt a secondary infection either with the way it's not just bobbing at the surface.. thats to big to be all air in there.. and thats sad and disgusting. or possibly parasites as well.. ugh... poor things gasping like crazy. and then tapping on the glass to get it to move around.
Not swim bladder disorder. If it was, the fish would either drag itself around the bottom or float upside down which is where slings come into play to aid their swimming abilities, neither of which is the case with that goldfish. It is clearly dropsy and the water in there is just as ridiculous.