Actually I initially had it outside the cage in the slot that's built in for the t8. The material used in animal plastics enclosures is so thick that controlling the temp inside the enclosure from outside the enclosure is not possible. I had to set the thermostat to like 120 just to get the temp inside the enclosure to the proper temp on the warm side, and THAT I felt was much more dangerous than having the probe inside with the thermostat set to the proper temperature. It's under the substrate right up against the enclosure bottom, up against the corner, and has decor on top of it. There's almost zero chance of it getting peed or pooped on and absolutely zero chance of being dislodged. The water bowl is on the other side as well so no chance of that either.
With glass enclosures and tubs I 100% agree that the thermostat probe should go under the enclosure between the heat element and the bottom. But it's not possible to control the temp in an animal plastics enclosure that way.
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