According to Aquariums Knowledge, red spots on a fish's body are usually a sign that its environment is in poor condition. Whenever a fish appears to be ailing, a water quality test is in order before any medication is added to the aquarium.
Aquariums Knowledge sets forth several reasons a goldfish might have red spots. Most of the time, red spots are a sign that the fish's water quality is dangerously low. Red spots can be caused by parasites living on the fish's body, a condition that is worsened when a fish's immune system is affected by its environment. Red spots can also be a sign of ulcers. According to Dr. Johnson's article on fish care, the first signs of an ulcer infection are red areas or spots on the fish's body. Many fish that suffer from ulcers also suffer from fin rot, a bacterial infection often caused by poor water quality. The Goldfish Tank explains that a fancy goldfish needs a minimum of a 20-gallon aquarium to thrive. A common goldfish needs 30 gallons. Because all goldfish are notorious for high waste production, it doesn't take long for toxins to build up in an aquarium that isn't large enough or that lacks a powerful filter.