Im having exacly the same problem... here in the uk its starting to get very cold overnight.. i had one heat pad originally and a 40 watt bulb during the day.. just the mat overnight but ive had to add another heat pad, & ive been putting the heating on high before going to bed just to keep it in the early 70s overnight.... ive added extra moss so they can bury if need be & have taken to filling their pool with warmer water when i change it every morning.....