Might be a Sick Goldfish

It looks like I have a sick or wounded Goldfish. See the nasty white spot at the base of the tail on this big Goldfish in who lives in the small pond. I noticed him hanging out in the corner and on closer inspection, I saw the spot. When I tried to catch him to get a close up view, he swam away without hesitation. And he stayed out of camera range so the second picture is one of the other pond residents who doesn't seem to have any spots. I did a little research and the spot may be a water quality problem. Tomorrow I'll buy a test kit if I can find one and see what the water is like. If it is just a water quality problem, it should be pretty easy to fix.