You can use any sort of under tank heater with a thermostat. UTH costs 20-30.00 and a thermostat about 30 if you go with hydrofoam. If her temperatures aren't up, that is almost certainly the source of your issue. They NEED temps between mid-80s and 90 to digest. Otherwise, the food just rots in their stomach.
Warmer ambient temperatures CAN work, but if it's getting as low as the 70s, then you've got a problem!