I have a question, I am planning to set the tank up this weekend so I can see over the next week or so how the temps do. To keep people from having to read back through this entire thread I'll give a brief recap. I have a 20 gall on long tank. I have 2 UTH's, one to go on each end set for the high and low temps. I am going to put the thermostat probe between the mat and bottom of the glass. My question is should I let it heat up and use my heat gun to measure the temp of the glass before substrate is added, or do I want to add substrate and then set the stat to reach the desired temps when taken on the surface of the substrate?