A white spot or spots on goldfish is a disease called "Ick," also known as white spot disease. The white spots look like grains of sugar or salt sprinkled on the fish but are actually parasites feeding on the fish. This disease is contagious and potentially fatal.
White spot disease is the most common fresh water fish infection. Because goldfish are the most common aquarium fish, people often think of the disease in relation to them, though all aquarium pets are vulnerable to the disease.
Pet owners who notice any white spots on their fish should take immediate measures to treat them. A delay in treating fish with white spots results in the death of the fish, and the disease spreads to other fish in the tank, eventually killing them too.
The white spots are parasites in their feeding cycle. They are actually eating the fish, and when they are done eating they simply fall off and adhere to parts of the tank. The parasites multiply quickly.
These parasites are usually introduced to the tank by a new fish or some new equipment that is introduced in the tank. Disinfecting a tank requires raising the temperature of the water to speed up the breeding of the parasites to kill them all faster, checking for ammonia and nitrate levels in the water, and using chemicals to kill the parasite.