I remove all of the dirt and tape my probe directly to the glass on the bottom of the tank, inside the tank. I know a lot of people are going to say something about how I shouldn't have tape inside the tank because it can get stuck to a snake, but it's packing tape so it's really strong. I would be surprised if a snake had the strength to pull up the packing tape even a little bit. Depending on the type of thermostat you have, the temps will fluctuate a little. I use the jumpstart heat mat thermostat, and when the mat gets up to the temperature I set it at, it shuts the mat off. Once it drops down to something like 88, the power kicks back on. Other thermostats simply regulate the power and those will keep your temps more constant.