Had a new vinyl liner put in last year and when took blue solar cover off for even a day would get black spots in shallow end usually in egg shapes and vary in size from one inch to 12 inches long. They are back again this year and we had to replace solar cover due to age and went with heavier duty clear. Using a salt generator also and have for about five years. Taken samples to two different places and neither recommended solution has worked. First was a tablet to put in skimmer and didn't do anything. Second said we had high phosphates and that was eating up the chlorine,so we shocked with liquid 4 gallons per day for 3 straight days. Still there and now she is saying Baquacil MIGHT be an alternative as she had other customers that tried it with success. Welcome any help or advice.