ric is right, you can get metals staining in fiberglass pools from many sources, not just fill water. Fiberglass just tends to stain easily. You can manage it just as ric says - with a sequestering agent. I would just balance your new water, and if you notice any stains, you can then lower your ph to 7 and put in a sequestering agent. You will then have to always add your matainence dose, especially when adding new water. Don't worry it isn't hard, and getting rid of stains if you get them isn't hard either!