can't get scum off of tank

[scrager]

I've got an all glass aquarium 125G that was once freshwater. the person that owned it let the water run low and deposits were made in a ring around the top around 4 inches wide. it's a white scum.

So far I have tried vinegar, amonia, greased lightning, and a magic eraser both with no-scratch scrub sponges and with razor blades. I've also tried steaming it off and then scraping with a razor. Nothing is taking this off easily. I've gotten one place clean, but it requires major razor scraping and took me about 2 hours to get a run of about 6 inches clean.

any other ideas as to what this scum might be or how i can remove it?

thanks

[Fragmented]

Easiest way I have found to clean a used tank is muriatic acid. Just fill the tank with water and add muriatic acid. Leave it overnight and rinse in the morning. Before rinsing add baking soda to neutralize the muriatic acid. Do this outside of course.