I managed to get a good clean on a 120 gallon 6 foot long glass tank by using a wet Scotch Brite, the blue one. The one I used said it did not scratch anything, which is important since Scotch Brite also makes some very abrasive cleaning sponges. I would just wet an area and let it soak for 10 minutes or more to soften the deposits a bit then step into the tank and start scrubbing. Yes, a tank that big lets you step inside to work, in socks only, no rough shoes please. It takes time and some elbow grease but it works. Back in my youth, I would have used salt as my abrasive. It also works fairly well but requires a really good rinse after the glass comes clean. After all you are not setting up a salt water tank.
