I bought a used tank off a fellow member and the overflow box on it was covered in coraline as well as parts of the glass.
I just used this thing,
http://www.ha-ra.ca/product_cloth_nano_kit_hh.php
We have a few of them laying around the house and they are good at other cleaning job like when let a pot boil over and burn milk product or plastic to the glass stove top... (oops) so I thought I'd give it a try on the tank. Little water, little strength behind it and it all came out nice and shiny. But its not exaclty something you can run to the grocery store and buy, so I dont think my post is all that helpful... lol.