I acquired a 100-gallon tank that has been 1/3 filled with water and sitting there for more than 2 years. The tank was really nasty. I cleaned it out and the only thing left is this white ring around where the water level was. I've tried vinegar (diluted and full strength), oxiclean, bleach, hot water, CLR, and finally muriatic acid (diluted and full strength!!). This stain won't budge. Not even a little bit. Is there any other way or any other chemical I can use to get it off? I'd appreciate any help. If not, I might have to bite the bullet and set it up even if it has a line going across the tank (less viewing pleasure, but more comfortable inhabitants).