I know it sounds crazy, but the fine steel wool you buy at the grocery store is the best thing in the world for cleaning tanks. It's softer than glass and will NOT scratch the glass. I discovered it myself as a kid and have been using it since then to get off the nastiest calcium deposits. I've even used it on ADA tanks, which I treat as precious, holy relics.
Oh, and it helps to keep the steel wool nice and wet. Also, I only use about a quarter size piece of it at a time and give it a little elbow grease. Once you're done with it, throw it away, as it will rust quickly and you probably wouldn't want to use it after that. Once bag of steel wool should last longer than you can keep track of it. I always lose my bag or leave it somewhere before I run out!
Try it! I promise it works and will not scratch your glass. Test it on the bottom of the tank first if your scared!