Hi my pool is 25000 gal with vinyl liner. It's abut 12 years old but this is my first year with it and 1st year with a inground pool so this is all new to me. My water is crystal clear but since I opened it I was getting residual light brown/tan residue scattered in deep and shallow end near the returns. I thought it was DE from my filter so I had pool co come out and clean and inspect my Hayward filter. After inspection they verified all the internals were good and no tears so that eliminated that possibility. So for a while after that the residue stopped. As of last week its back but now its actually black. As soon as I put the brush to it it dissipates. Gets a little cloudy and gone. Besides these 2-3 patches the liner in some areas is coated in black also and again as soon as I brush it, it goes away. As for my levels. So obviously with all this happening I decided to check my levels and noticed my alkalinity was at 30! And PH was like 6.8. Everything else was ok. I also checked for phosphates and they were 0. Just this morning I added about 25 lbs of alk plus to get that to the right level and see what happens. Does anybody have any other suggestions? I am very new to this and would appreciate your help. If I missed something which I probably did let me know.