You can put the probe on the outside of the tank between the under tank heater and the bottom, just tape it to the bottom of the tank. I also put the probe in the tank taped to the bottom but it can get wet, not good. And if you switch to a rodent tank they will chew the probe, happened to me before I caught it. Once the probe gets to the set temp it turns off the heater until it cools to a certain temp. If the probe is not in the hot spot it won't get to temp and your heater will run on high.