Intalled pump and got water circulating a week ago but left winter Loop Lok cover on . Check water and FC was at 9.0 and water looked clear .Have been running roughly a week as in Pa still very cold .Today took off the winter cover and water was clear .Vacuum and got everything up but noticed that we had a lot of stain or something on vinyl liner. We have been on bleach for three years and pool is a 23000 gal inground with a DE filter . We tried brushing and it did not work so I got out a pad (Scotch ) and use it on a applicator and it is removing the stain and it appears to be black but it will be a lot of scrubbing . I did shock before getting out the pad. I will post readings after checking water in morning but PH was 8.0 the other day . Chrlorine FC was at 5.0 but CYA appeared to be below 30 .Have not added anything else like Poly 60 yet . Thoughts -next steps