Hermit crab tank heaters are needed if you wish to maintain the proper humidity in your hermit crab tank. As you will see in this picture, I have given you an example of the boxing of one type of heater of which once it reaches it's highest temperature (Of which is an appropriate temperature for hermit crabs, mind you.) it will then proceed to stay at that temperature consistantly. The only way to deactivate it being to unplug it from the wall socket.

The hermit crab tank heaters' box that I bought for my hermit crabs.

Basicly, it sticks to the OUTSIDE of the tank (Only use it with glass tanks, no acrylic or metal or plastic tanks, as they may melt or damage the heater.) using an adhesive. Do not use it with water-filled tanks or get water on it as this might result in an electric shock. Do NOT use it inside the tank, as this might injure your hermit crabs. Only combined with regular misting though does it help your tank stay humid, so don't forget to mist the tank!