HCA: Hermit Crab Association

New to this and want to do things right for my daughter’s crab. I have a 10 gallon tank and need to buy a heater. I would like an ultratherm but I can’t figure out what size I should buy. I have read a ton on this subject but I can’t make a decision on what I should go with. Her bedroom where the crab will live stays about 70 all year long. Any help you could give me would be appreciated.

Ultratherm heaters can be a bit expensive but are worth it.
Buy one by the physical dimensions, not by the number of gallons it says it will heat. You want it to fit pretty much across the entire back of the tank. And much of the air above the substrate.