Many times its's just a scratch that will go away with salt and extra water changes. However I think your discus looks sick in the photos. Nitrates are high at 20 also. I keep my nitrAtes under 5.
First line of defense is always extra water changes and salt @ 1 or 2 tablespoons per 10 gallons. If your fish is not better after 3 or 4 days I would suspect an internal bacterial infection.