Marty, If you keep your alkalinity in range - 80 - 120, and your calcium in range - with fiberglass I keep mine at 150 - 200, you can keep your ph as low as 7.0, and it won't damage the heater. If you have a fiberglass pool that is having staining issues, then I would keep the ph at 7.2, and no higher than 7.4. Keeping sequestering agent in at all times.

Gerri, Take your ph down to 7.2 - with the additional sequestering agent, the stains should lighten up till you use the metal magic. Let me know how it goes. I haven't used it yet. I just got home from vacation and am having company, so I won't get to it for a while.