Spot found on Goldfish......help!
It looks to me like the beginning of an ulcer. I would stop feeding your fish for a few days and make a one third water change from the bottom of the tank. You should notice a difference in the colour of the ulcer in a few days. If the tank has been set up for a long time and the gravel isn't clean, it is possible that the immune system of your fish has been compromised and it can't fight off the infection.
If the infection continues to get larger, I would check out your filtering system to make sure that it is large enough and make another one third water change from the bottom of the tank and don't be afraid to cut the feeding of your fish way back. Remember, these fish aren't getting the exercise they would normal get if in a larger pond so they don't require large amounts of feeding like pond fish.
Yours Koily, Lorraine