I had my pool replastered last year and this year upon opening there is a tan colored stain covering about half of the deep end. I have the PH at 7.3 and have maintained shock level 30 for about 4 days and scrubbed the stain to no avail. Right now the TC is around 5. I have tried putting vitamin C in a sock and holding it against an area of the bottom for 30 minutes and don't see a white spot in the stain like it should if it were a metal stain. Any ideas?