Your goldfish is suffering from Swim Bladder Disorder (sbd).
The swim bladder is responsible for a fish being able to swim upright and maintain its buoyancy. When it becomes blocked, infected or too full of air it ceases to function properly.
Many types of fish are highly susceptible to this disorder mainly because of the odd shapes of some species, genetics and poor breeding practices.
The way to treat sbd is to begin to feed the fish canned peas without the husk/shell. Feed 2-4 peas twice a day making sure to clean out any uneaten portions. Peas are well known to clear out a fish's swim bladder. It's best to feed the pea diet exclusively. If the pea diet does not work then try the pea diet with intermitent days of fasting.
In addition to the pea diet you can try treating the fish with a medicine called Maracyn-Two (USA) or Interpet Aquarium 13-Swimbladder(outside of USA) which could prove helpful in curing sbd as some cases are caused by bacteria. Follow the directions on the product label for dosage amounts.
It would also be helpful to not fill the fish's tank up to the top with water leaving about 2-3 inches/5-8 centimeters from the top free as this will lessen the amount of water pressure on the fish.
Please keep in mind that sbd can sometimes be very difficult to cure and is one of the
most common problems goldfish keepers face.
Best wishes and please let me know if you have any questions.
2118 satisfied customers