Dave--I should think the humidity question would depend a lot upon the ambient humidty in your home, as well as the size of your tank. Larger tanks would have more "micro-environments," while something like a 10 gallon 10% open, would dry out pretty fast in my area. I like to see my frogs out and about, comfortable in all areas of a tank, rather than seeking out comfort zones, which might mean hiding in leaf litter or down in bromes and spending little time in the open or climbing around the higher reaches of the background.
On the other hand, that's probably why you are able to grow so many different mosses so well, and I have no real success with anything but Java.
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