Wondering what may cause this? My numbers are right where they should be. It's just around the water line, and slightly above it. I have tried magic eraser, vinyl cleaner (it contains enzymes), pure chlorine, and diluted muric acid, none of which seems to remove this. Is there anything else I can try?
The liner is one year old and I also added borates this year.
My next step is to try a vitamin C tab on the stain.