Not sure what you are using for water, but most products marketed for hermit crabs are not safe. A lot of us use a product called Instant Ocean which is what aquarium hobbyists use to make saltwater aquariums. It has the essential trace elements that hermit crabs need.
You need a dechlorinator that removes chlorine, and chloramine as well as neutralizes ammonia and heavy metals. A lot of us use a product called Prime. As far as salt, you need marine salt, to mimic the ocean. You have to use the dechlorinator for both freshwater and marine saltwater. Both pools should be deep enough for them to fully submerge in and safe enough for them to get out of.
Water pools: Bubblers definitely do help boost the humidity. Double dish bubblers:
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For food: Hermit crabs can eat a lot of the same things that we eat. Just check to make sure the item is on the safe list. Foods should be organic when possible. Hermit crabs are sensitive to pesticides and fertilizers.
You also want to provide one or more calcium sources at all times. Examples would be Cuttlebone, Organic Eggshell, Oyster Shell, Lobster Exo, Shrimp Exo.
Make sure you cover all aspects of food pyramid
Safe food list:
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Unsafe food list:
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Food pyramid:
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Sponges can be used as a food source. Squeeze all of the water out of it and put it directly on the substrate. Do not place them in the water bowls.
Hermit crabs are also known as tree crabs. If your yard isn't sprayed with pesticides and fertilizers, and your city/county doesn't spray for mosquitoes (you would have to call), you can use branches from safe wood in your backyard (Oak, Sweetgum, Maple) for hermit crabs to climb on. You would want to bake the branches to kill any bugs.
If they are eating that well, they are doing good!!