the lol was from a few years of keeping gold fish and not had a problem with feeding from the surface
i have seen problems with gas in the gastrointestinal tract though which can be corrected sometimes by feeding green peas
i suppose if the oesophageal sphincter was weak it could allow air into the digestive tract
or many commercial goldfish foods contain wheat and soy which can create gas in the colon caused by bacteria fermenting the starch or sugars found in them
digestion of these starches by animals requires an enzyme produced by bacteria present in the gastrointestinal tract producing hydrogen, methane and carbon dioxide
it is also possible that high nitrates can affect the vascular mechanisms for controlling the amount of air in the swim bladder