I had waterline stains on my fiberglass steps, they were sort of grayish greenish colored. Wierd thing is, we also have a fiberglass swimout, and there were no stains on the swimout. Anyway, I had read that someone tried the Magic Eraser, so I tried it and it worked like a charm!!! My question is, should I further investigate the cause of the stain (metals, etc?) or just be thankful that all seems OK? My numbers are:

FC and TC 5.0
CYA 50
PH 7.6
TA 140

I use bleach to chlorinate and shock, and trichlor sticks when out of town. Pool was filled May of this year, and I've had nothing but clear blue water all season.