One thing that really helps keep tanks humid enough is making sure you have a proper lid. If you use a wire lid try taping plastic wrap over some of it to keep humidity in. Just using a mister in dry patches works well. Just make sure you don't most too much, especially if your hermits are burrowing. The extra water collects on the very bottom of the tank.
Also, what kind of substrate are you using? I use eco earth, and it holds moisture really well. Lots of people have also mixed sand and eco earth.
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