What's your humidity and temperature reading?
If your only heat source is the heat pad, it probably just isn't enough for the 20 gallon. And just to check, you have it on the side of the tank and not actually under the tank, right? I think you need another heat pad - when you're getting one from the store, the recommendation is to ignore how many gallons it's for and go for the biggest size you can get that will fit along the back above your substrate line. You can use a heat bulb but they suck the humidity out a lot faster.
The most highly recommended heat pad would be an Ultratherm, which can only be bought online but they're typically less expensive than a large one from the pet store. They're also more reliable, and can be safely insulated with aluminum foil and/or styrofoam to boost your temps more. One this size:
http://www.reptilebasics.com/ultratherm ... uth-6-x-23 would most likely work well for a 20 gallon.
What kind of lid is on your tank? If it's a wire/mesh one, you can seal it up with saran wrap or tape foil over it to hold humidity in. A moss pit can help raise humidity, as well as installing a bubbler in one or both of your water dishes.
If you could, copy and paste this template and answer the questions and we can get a better idea of your setup and hopefully offer some more advice:
viewtopic.php?f=27&t=46102
I haven't seen my crabs out in a long time I only saw one last night. They eat their food because I notice it goes missing or is bitten off and they definetly use their salt water and freshwater dishes. I'm very upset I didn't have this issue with my ten gallon 
