It should go between the heat mat and the bottom of the tank. You'll need to play around w/ the temps on the thermostat to get the temps you want on the surface floor. Keep in mind, you'll lose some heat due to the heat needing to travel through your enclosure (i.e. glass, pvc, etc..) so more then likely your thermostat temp will be set to a higher number then where you want your hot side temp to be. Mine is set to around 100 to achieve surface temps in the low 90s.