If you can attach some pictures of the goldfish with the problem , that would help greatly in diagnosing it. 

Can you give some additional info about your current setup including  aquarium size, filtration type (if any), how many fish and their size, how often you change the water, how often you feed your fish, any new fish or plants added recently,  and do you use a water conditioner?

If you have a water test kit that shows the amount of ammonia , nitrites, and nitrates in the water, that would be helpful as well. Poor water quality is often the main cause of many goldfish illnesses.