You could mist the tank more often or switch to a better substrate that actually holds humidity. No offense, I hate aspen, it always gave me humidity problems. I use eco-earth expandable dirt, you put it in a bucket of water and the dirt will expand to fill a 40 gallon breeder tank. If you do not want to be bothered with expanding it, you could get the loose coconut husk substrate.

You will eventually need to mist any substrate, but with eco-earth or loose coconut husk substrate, you mist it way less than aspen.