I made a waterer of 4" vertical pvc with two cup auto waterers for our ducks and chickens. I want a low watt water heater I can drop in there that won't get too hot and does not have to be submerged, in case they run out of water. I may wrap the pvc in insulation. An aquarium heater would be great, but they have to always be submerged.
What can I use?