My thermometer probe is in the tank on the glass and it reads 94 degrees F. My thermostat prob is in between the glass and the UTH on the outside of the tub. I used to have it on the inside, and the temp would fluctuate from between 86 and 100 degrees F. I don't know why, but that's what it would do. Now that it's on the outside, it's dead on. I also found that the temp of the thermostat wouldn't read correctly before, either. There's a dial on it and the temp was never what it said it was supposed to be. Now it's almost dead on. I'm going to do what works to keep my temps on, and if that means not following the directions that came with the package, that's what I'll do! I was constantly adjusting the temp up or down a few degrees. Now I don't have to touch it! It's perfect!