One option for heating pads is a thermostat. Basically, it's a probe that goes between the heat pad and the glass and you set your desired temp. The thermostat will turn the power to the heat pad on and off to keep it at the temp you want it. You can usually get one for about $30 on Amazon.
Also, you may already know this but be sure to put the heat pad on the side of the tank and not the floor.