My 75 I started with had a TON of coraline and hard water "scum" on the glass that even the razor blade wasn't too happy with... Bought a bottle of Muriatic acid, some gloves and a sponge... dabbed the acid on the stains, let it sit 10 minutes and then wiped it down with the sponge soaked in acid, and it looked as good as new. Threw a box of baking soda into the tank and filled it with water to neutralize the acid, rinsed it a few times and it was perfect!