I don't think any UTH can be used safely with a substrate layer that thick. to get the top of the substrate hot enough the bottom of the tank will need to be far too hot. I always suggest a substrate layer 1/2" thick or less (the thinner the better)
If your room is 80 degrees and you have a UTH providing a 90 degree hot spot no additional heat sources are necessary. (When I was living in Clearwater and still using glass tanks I found that I didn't need to use a heat lamp, a UTH was all that was necessary)
I will add that i will be back in Clearwater this coming weekend (July 13-16)
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