I have a 55 gal glass, but the substrate doesn't matter. In order to keep humidity up, i have to cover the top with a wet towel. I also have a waterfall (great to keep humidity, but not fun to clean, and you have to do it often). I also have a cup with some paper towels in it to help (which also has to be cleaned at least once a week). I just switched from repti-bark to aspen, and neither seemed to do big changes to the humidity. I can't wait to get my rack built.