Need some help, first started with goldfish with 1 white eye, acting fine. Then others started getting white eyes, spreading. One goldfish with lump on his head. All within last 2 to 3 days. All water quality negative, have added salt to water at 0.25%. I do water exchange of 150 gallons weekly now in a 2000 gallon pond. Never had any problems before. Pond not heated, have had cold weather last month, water in pond at 35-45 degrees. is 18 inches to 36 inches deep.