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.
Plexiglass/lexan can be purchased at most hardware stores and be cut to size for free. This can be placed on top of your existing screen lid for stability.
Coroplast is another good option for a lid. This can be placed on top of your existing screen lid for stability.
In the short term, you can use plastic wrap over the screen lid. Some people use cardboard and wrap it in plastic or tape.
Your tank should have two types of water bowls, salt and fresh. Use bowls that are wider than they are deep but still accommodate your largest crab and allow it to submerge. This increased surface area will create more humidity. Water dishes may be placed next to your heat pad or beneath your overhead light to produce additional humidity. Adding bubblers/air stones to the water pools will boost humidity.
Crabs love moss! Placing moss anywhere in your tank and keeping it damp will raise your humidity, especially next to the heat pad. Rewet/replace as needed. You can buy moss in bags or compressed bricks. Sphagnum moss is a favorite and it holds up really well in the crabitat long term. If you can’t find sphagnum, look for Hiawatha or sheet moss, terrarium or frog moss, natural and un-dyed. Moss does not need to be cleaned or replaced for cleanliness.
Wood: Choya, cork bark and other safe woods can suck up the humidity in the air but once saturated can help maintain it.
Things to avoid
Soaking your substrate is not the way to raise humidity.
SLIGHTLY moistening the substrate next to the heat pad is ok. Water logged substrate is not safe for molting hermit crabs and will encourage the growth of lethal bacteria.
Foggers: Foggers and misters can easily flood your tank and lead to lethal conditions for molting crab or trigger a bacteria bloom. Use with caution.
Sponges: 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.
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 and some plastic wrap on top of the towel. The towel must be kept wet and it must have plastic wrap over it or as the towel dries out it will absorb the moisture from the tank. This is not a long term solution.
You can also mist the tank with dechlorinated water near the heat source. This is not a long term solution either as the humidity will fluctuate too much between misting to be suitable. You also risk flooding your tank by over misting.
Originally posted at All Things Crabby The Hermit Crab Care Blog
Some lid options to consider: