Quote Originally Posted by CarlD View Post
I don't know the chemistry of FG pools too well, but I cannot imagine how calcium interacts with glass and the resin used to bind it. I'll let Ben and Chem_Geek work that out. People forget: Fiberglass is actual glass--super-thin threads of it.
Actually Carl, I've done quite a bit of research on this and have discussed it some with Richard. There is some evidence that the calcium (actually, the harder water) helps prevent cobalt spotting (black spotting), which is a problem with fiberglass pools. (Cobalt leaching from the gelcoat. Cobalt is a component of the resins used in fiberglass.). Cobalt spotting is probably only second to iron staining as a downside to fiberglass pools but both seem to happen much less with higher CH. (For example, my own pool is filled with soft water and I had frequent problems with iron stains, usually requiring an ascorbic acid treatment every 6 months. Once I raised my CH to around 350 ppm about 2 and a half years ago I have not had ANY staining problem and have not needed ascorbic acid.) However, unlike iron staining, cobalt spotting if often difficult or impossible to remove. There has been discussion about this on a different forum, btw.