Thanks for the info folks! I have tried muriatic acid to no avail. I think its beyond a hard water stain and more towards some sort of oxidation. The tank is a 175 bowfront that I picked up used. I've invested a lot of time in building the stand and 55 gallon sump for it before i realized that those aren't your everyday water stains. I'm just trying to exhaust my options before I give up on the tank. Cerium oxide in that form isn't anymore toxic then any other particulate natural material (ie: silicate dust from sand) Of course care must be exercised to avoid inhaling the dust but there shouldn't be any residual effects on the tank since it employs a natural abrasive characteristic to work rather then a chemical one. They use the stuff to remove fine scratches on windshields and to polish glass. I figure since I can't seem to remove the oxidation itself from other means, I'll try this to polish it out. I found some on Amazon and will post the results. I'm also waiting to have my paypal account re-activated(from inactivity) so I can sign up as a charter member. Wish me luck and thanks again for the input!