The Importance of Temperature for Hermit Crabs
Temperature within your crabarium should be monitored on an ongoing basis. I suggest you purchase an adhesive temperature strip which should be placed along the bottom of the crabarium, parallel with the bottom of the tank as it is important to guage the heat at substrate level.
The temperature in your hermit crab's cage should be between 70o F (21oCelsius) and 80oF (27oC) The ideal temperature to hover about is 75oF (24oCe). Try not to let your crab's home fall below 70 degrees for very long or your crab will become inactive and may perish. If your crab overheats you may see a brown discharge. This means that your crabitat enviornment needs to be re-assessed.
If it is winter or below 21 degrees in your home and you do not have an undertank heater or other heat source an alternative action is to take your hermit crabs out of their crabarium for some play time and find a warm spot for your crab's tank to spend time during the day, such as a window sill or a sunny area outside. Remember to bring it in in once it starts to get cold. Return your hermit crabs to their crabarium and the send should be just warm and a lid should be placed to help trap the warmth in.
If the temperature falls below 21 degrees frequently, you may need to purchase a under-tank heatpad but be careful that you do not overheat your crab. If the temperature rises above 27 degrees (80oF) add more substrate (sand or gravel, etc) and experiment until you have the crabarium temperature falling within the hermie-friendly range of temperatures. A 24 hr timer is also a good investment as it will turn the heater on and off throughout the day. Keeping it on during the cold hours and turning it off when you set it to.
"Of equal importance is humidity control. In order to breathe, your [hermit] crab's lungs must be kept moist to exchange carbon dioxide for his much needed oxygen. Unlike human or mammal lungs, his lungs do not expand to take in air, but rather extract oxygen along the external surfaces of their breathing apparatus. Therefore, if his atmosphere becomes too dry he will suffocate. Always keep plenty of water in his bowl. This will assure him of an adequate level of humidity for proper breathing... If the air in his house[or tank] is allowed to becometoo cold or too dry, he will become inactive, and if not corrected after a period of time, he will die!" (Merv Cooper's Crazy Crab Handbook)