Jewlers rouge, a buffing compound might take it out if its not too deep. If it is only on one side of the glass, use it as the back. It can be hidden by plants or other decor. I would sanitize the tank afterwards with a bleach solution, 1 part bleach to 19 parts water. Let it dry in the sun for a few days and rinse well.

Thank you for the Jewlers rouge suggestion, I have some and will try it tomorrow. Funny thing is, I can't even tell if it is on the inside or outside, though I would bet it is on the inside to have such a problem. I can't imagine what animal they had in it. It is on all four sides of the tank. I already sanitized it with the bleach, and it was out in the sun for three days.

Thanks for the suggestion, can't wait until tomorrow to try the rouge.

try wetting and scraping with a razor blade

Hi
I've used very fine steel wool.Though it's a lot of work it works!! lol Removed hardwater spots as well as calcium build ups without scratching.
gary

Isn't there a product called CLR for calcium build ups?

Ianna