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Swollen eyes in goldfish is often the first symptom of 'dropsy' which is a disease of goldfish that can eventually result in kidney problems and illness- it can be caused by virus, bacteria or fungal infections, or even environmental issues such as poor water quality.
As a precaution i would isolate this fish in a 'hospital tank' keeping the water temperature in the high 70's- also add 3 tsp of salt to each gallon of water, ensure that a pump is used to increase water oxygenation.
If this fish goes off its feed , then start antibiotics such as Maracyn plus or Maracyn 1 and 2.
If the swelling goes down then the fish can go back in the normal tank