The first step to creating and maintaining humidity in your tank, is a good lid. A screen lid alone is not sufficient. A glass/plexiglass/lexan lid, cut to fit is the best option. In the short term, you can use plastic wrap over the screen lid. A small opening should be left for air exchange. The size of the opening may have to be adjusted until you find the right spot to maintain your humidity. Plexiglass/lexan can be purchased at most hardware stores and be cut to size for free.
Soaking your substrate is not the way to raise humidity. SLIGHTLY moistening the substrate above the UTH is okay.
Your tank should have two types of water bowls, salt and fresh. Use bowls that are wide and shallow. This increased surface area will create more humidity. Water dishes may be placed slightly over your UTH (under the tank heater) or beneath your overhead light to produce additional humidity.
Crabs love moss! Placing moss anywhere in your tank and keeping it damp will raise your humidity. Replace as needed. You can buy moss at the pet store in bags or compressed bricks.
What Moss is Safe?
Is Beaked Moss Safe?
Is Spagnum Moss Safe?
Don’t Forget the Moss
Repti foggers can be placed in the tank to create humidity as well. They cost around $30.00 and can be purchased at your pet store.
You can make your own with the directions here: Homemade Humidifier
If you have just discovered that your crabs need humidity and you are not able to fix the problem immediately you can place a wet towel over the tank. The towel must be kept wet and this is not a long term solution. You can also mist the tank with dechlorinated water. This is not a long term solution either as the humidity will fluctuate too much between mistings to be suitable.
Sponges were once considered a staple in the hermit crab tank but there are better solutions to the humidity issue. Sponges placed in the fresh water will raise humidity. Sponges should be changed out daily, sterilized and replaced with already sterilized sponges. Sponges placed in the salt water will become nasty fast. Due to the upkeep of sponges it’s easier to go with moss.