Currently, I have my four crabs in a 20 gallon tank. For the first little while, I was having troubles keeping the heat low enough (it kept spiking into the 90s), although recently I've been having the opposite problem. I've been having difficulties getting it to hit the 80 mark. In my 20 gallon I have two UTH: the largest one is wall mounted and takes up an entire side, and then I have a really small one stuck to the bottom of the tank (put this on years ago when I didn't know that you shouldn't place them on the bottom; now I only use it when I really need the extra heat and know for sure that there is no crab around it). I also have a hood light.
The new tank I'm getting is obviously much larger and comes with a screen lid instead of a hood lid. So I'm really at loss at how I'm going to heat it properly.
Any suggestions? Also, does anybody have any suggestions about how to safely remount the large UTH onto the new tank, or is it permanently stuck to the 20 gallon?

Thanks everybody!