The tank heater pads actually do have a built in thermostat.
With the control panel switch on, the thermostat turns on at 45⁰F off at 64⁰F.
Having said that... I've bought tank heater pads for other projects. One didn't work at all. The built in thermostat was bad. How would you possibly know if your pads are all working unless you have an accurate BVM and watch the current draw as you turn on/off the heater pad in question (assuming the temperature was below 40⁰F).