I too use aspen, but make sure the probe is measuring the temperature of the class and uth. If the probe were place on top of the substrate and the animal burrowed it might be to hot. I have had not had a problem with any of my tanks. By covering a good portion of the top and misting the aspen daily my humidity level stays at a good range. Others with more experience can offer better advice as far as t-stat probe placement goes. Mine is in the tank held against the class using aquarium sealent and a small piece of wood to hold it against the glass. Make sure you do not cover the probe itself. Others feel it safer to place the t-stat probe directly on the UTH, between it and the class.