I read something about staining on fiberglass pools, I copied the Paragraph:

"Fiberglass has a negative electrical charge and can attract positively charged metallic ions, causing the development of a stain. The periodic addition of a metal treatment should help negate this effect and help keep pool stain free. "

It would explain why fiberglass pools stain more easily than other pools, and why I can't figure out where the stains in my pool come from. I will try to find out more about this too --and if anyone else knows anything please help!