HCA: Hermit Crab Association

I'm not sure that Creature Soil is appropriate. Most of us use a mixture of 5 parts play sand to 1 part Eco Earth. Like Hermiesguardian said, substrate needs to be at least 6 inches deep (or three times as deep as your biggest crab, whichever is deeper), and need to be moist enough to hold it's shape so the crabs can make tunnels and caves to molt in. This also helps with humidity. Not sure if the Creature Soil can be moistened like that.

What is a 'tank heater'? An under tank heater is best - mounted on the back or side of the tank, never underneath.

good news: tank humidity is perfect right now, and it seems to stay good as time goes on.
bad news: tank heat could be better. Current heat is about ~69 degrees.

It's 3 am and I have my little tank heater light on XD

definitely get a uth. www.reptilebasics.com or www.beanfarm.com

In the meantime, you can warm towels or a blanket in the dryer (no dryer sheets), and then drape it over your tank to help with heat.