to the forum for starters! Lot of people with good info on here.
Most people, and myself use Flexwatt heat tape under the tubs. Its thin and comes in several widths and wattages. Some use UTH , but what ever you go with, you will need a thermostat of some kind to control the level of heat. Most tape the thermostats probe right onto the Flexwatt(or other heat source if possible) and place a thermometer probe on the bottom of the tub right over the heat source. The hot spot should be around 90-94 degrees and the cool side 78-83. But a thermostat is the 2nd most important thing besides the heats source.