This is what no one says about tubs, you will need to keep the room the tubs in warm via a space heater, oil heater or some other external heat source since as you found out, you cant really heat the ambient air in only the tubs very well with colder outside ambient air. me personally, im sorry but i don't use tubs, i use glass tanks for my babies with just CHEs and they do fine. Then I'll switch them to PVC cages when they outgrow the glass tanks. Maybe someone has some idea other than a portable room heater.