I have a 15000 gal fiberglas pool. I noticed a ring around the water line that does not just wipe off. I vaguely remember reading something about ph level and chlorine level causing such a ring. Does anyone know what I am talking about and is it high ph and lower chlorine level? Yesterdays readings were FC- 3; CC-0; TC-3; ph 7.6-7.8 (color variation kills me) TA 80; CYA-50; TCAL- 240 water temp 86. Otherwise my pool is clear blue and beautiful.