HCA: Hermit Crab Association

I'm setting up a new habitat for 2 small hermit crabs. I know I should install the Ultratherm UTH on the back wall above the substrate line, but I can't find anything that says whether it should be on the inside or outside of the tank? What is the general consensus? Thanks!

It's actual missing limbs, appears to be the back legs 1 on each side. No change so far, it hasn't moved and still doesn't smell at all. I took all of the recommendations so far expect for the larger tank. Do you have an recommendations on which tank? If I go up to a 20 gallon I assume we are talkin...

Thanks for your response. This crab was and is always on the surface. I think I need a new tank lid/hood to keep the humidity up when it's not summer, do you have any recommendations there? Also do you think I should add more substrate right now before I know for sure the crab is dead? I'm hesitant ...

1. What kind of substrate is used in your tank and how deep is it? A mix of playsand and coconut fiber. About 3-4 inches deep 2. Do you have gauges in the tank to measure temperature and humidity? If so, where are they located and what temperature and humidity do they usually read? Yes generally tem...