Hermit Crabs: Sand or Wood Chips?

I'm having trouble keeping my tank moist. I currently have two water dishes (one salt water one regular), moss, fake foliage, some shells, a branch, and a log cut in half used to hide. My substrate is coconut fiber and wood chips but I was thinking that maybe it's the substrate that's causing the humidity problem. I also have a humidity gage and temp gage and is I don't constantly mist my tank the humidity drops down to 20. Any help?