The probe is supposed to be between the glass (outside) and the UTH. This tells the UTH when to raise/lower it's temps. Once you have that dialed in (it takes a little time, trial and error) the thermostat basically does the work for you. The UTH is ONLY to provide a hot spot, not for ambient temps. This is where your heat lamp comes in to play. The heat lamp will be there for ambient temps. As you have it now, it runs full blast the entire time it's on. So, as room temps rise and fall, so will your enclosure temps. Setting the lamp on a thermostat allows you to se the temp and the tstat will do the rest.
Once you have it set up correctly, you just monitor it to make sure it's not malfunctioning.
If you search in the search bar for 20 gallon glass setup its right at the top of the list
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