Are you using an UTH mat? If so, sandwich the probe between the mat and the bottom of the enclosure then use the metal HVAC tape to secure the mat around the edges. Place your thermometer probe on the inside of the enclosure over the hot spot. You can then adjust your thermostat based on what your thermometer reads.

This is one of my QT tanks, looking inside of the tank. I do the same with my T8s. Never put tape over the probe itself: