I always use a 10% +/- solution of muratic acid and tap water. Larger tanks I spray it on and let it sit for a bit then anything on tank glass will wash or wipe off with very little effort. I usually just do one side at a time on larger tanks. Just make sure to use this stuff outdoors as it puts off an odor you don't want to breath and remember it is an acid so handle with care and always add the acid to water. It can also be neutralized with normal baking soda. If you have a smaller tank then just fill with water then add acid and I also put in a power head or something for flow but not necessary. You can also use straight vinegar but I never have any luck with the stuff cleaning very well or quickly and can't stand the smell so I use the muratic acid solution for most all my cleaning.