We have a 16x40 fiberglass pool had it covered for the winter when we opened it color was green it is 5 years old we cleaned it cleared the water but have a brown stain on steps, sides and some on bottom have tried everything, lemon, magic erase, stain remover etc. We have had it tested for iron, copper everything we can think of and it shows nothing does anyone know how we can get it out. The only thing we haven't tried is to drain it and wash but I read not to acid was a fiberglass pool. Help!