Marie,
When I was researching for the type of pool that I wanted the biggest drawback to fiberglass that I seemed to come across was that it was prone to staining and it was becuse of the negative eletrostatic charge. Metal ions have a positive charge so it makes sense. Don't know if it's true or not either but I STILL went with fiberglass and don't regret it at all!!!!!! I also keep my calcium levels at about 150 ppm. Iwonder if running the CAl higher might help prevent staining since calcium ions have a positive charge also? Might be an interesting experiment and if you get the right calcium, not an expensive one....check out this thread and what I found out about calcium (Dowflake)
http://www.poolforum.com/pf2/showthread.php?t=340