I use one ZooMed heat pad under one end of a ten gallon tank. Works well for me. I buy then off Ebay. When it get cold in the house I use a 60 or 75 watt light bulb with a reflector on the tamk screen lid for extra heat. I live in Northern Minnesota where it gets every year to at least 40 F below and the house has chilly spots at night. I use a Zilla metal mesh cover. Be sure to use lock downs at each end of the screen.