For my 66qt tubs I use 11'' flexwatt. This is large enough that it covers 1/2 the bottom of the tub and youll be able to use the same piece as your tubs get larger. Id shy away from the heatpads in pet stores just bc theyre way more expensive than flexwatt... 11'' flexwatt is like 4$ or something... Rich will hook you up with everything you need.