My curiosity has been piqued by plucky71's thread here. Low CH in a fiberglass pool tends to lead to cobalt staining. Why is this? I presume it's not from dissolved cobalt in the water, or vinyl pools with low CH would suffer the same problem.
Is cobalt used in the pigments in blue fiberglass pools? If this were the case, I would think that people with white fiberglass pools such as plucky71, whoozer, or waterbear (so I hear; he was before my time) wouldn't have anything to worry about, but it is still universally recommended to keep a middling CH level. I guess my question is just: what is the source of the cobalt that causes the stains?