Some Corrections - 1 >ascorbic acid "does not remove the stains" that occupy the majority of the pool . (2) Confirmed again this morning that the HTH calcium hypochlorite "does completely remove the stain"
I went to work on the fiberglass steps this morning to conduct my test and thus am now based on everything you said confirmed it is a "organic stain " that is causing me the concern .
We have had a extremely wet spring in Pa after a extremely hard winter .The water table is as high as I can recall and water is laying in my yard places it never has in almost 40 years . Have to assume much water behind my liner on the inground pool .
Check readings this morning and update PH 7.4 FC 11.5 CC0 TC 11.5 Alkalinity 120 and Hardness only 100 . Concern is that CYA is creeping up with using the powder HTH as now at 60 and with history of chlorine demand really hate to use the HTH . Line is within a year or two of replacement and is badly wrinkled in spots so by no way is it "smooth" . I ended up with water behind the liner and when sucking the liner back and removing the water it was left wrinkled or high "PH" has taken toll?
So have so far super shocked and kept chlorine levels high. Brushed and vacuum ever day . Treated with Poly Quat 60 yesterday . Any suggestions other then brush and vacuum? I really like the results of the HTH in a sock and going after the dark spots as it really is like instant clean but again worried about CYA ? Waiting for your expert opinion