Dang i posted late on this one. If you are using a small Rubbermaid or Sterilite you shold have no problems creating a heat gradient with just the pad under half the tank. But if you use a larger one like my 90qt the pad will not do the job alone so a light could be used to make the heat gradient more defined. Later on down the line when you get some money saved up i would invest in a good temp gun. I have one from tempgun.com and it works great it was quite expensive but it lets you check the tempreture on any surface.