I used straight vinegar. Left it on for a few minutes then scrubbed off with one of those non scratch pads. Just make sure you don't pick up any gravel on the sponge or the glass will get scratched. For the areas that were really stubborn, I used aquarium salt and scrubbed. For the stuff that just absolutely wouldn't come off, I used a razor blade(very carefully). Then rinse, rinse, rinse.
Good luck