Since I opened my pool this year I have had some pretty noticeable stains through it its shell (fiberglass). Its kind of a dingy spotty grey color. I have scrubbed the life out of it and it still had not gotten any better. Is there some type of chemical I should add to help dissolve it? I assume not knowing exactly what stained it might make it more difficult to remove. But I was not sure if there is something more universal that might be worth a shot.