Review on 🦀 Zoo Med 4-Watt Hermit Crab Heater: Efficient Heating for Optimal Habitat, 7.3 x 4.2 x 1.8 inches, Lightweight and Compact at 5.6 ounces by Brandon Show

I thought this little heater would do for my 10 gallon terrarium but it isn't. The heater barely changed the temperature and now I need a heater in the room where my crabs live to keep them warm enough. It might work if you have a 5 gallon tank, but it won't do for a 10 gallon tank. FYI, the packaging also says you only need 1.5 to 2 inches of substrate to use as an under tank heater. However, a properly set up crab should be twice the size of your largest crab so it can burrow when it's time to shed. So if your crabs are larger I don't think this will work in a properly configured crab.