What size UTH are you using for the 20gl? As long as the temps above the UTH are around the high 80's you should be ok. You want to make sure that the surface temps over the UTH is what your temping as this is what will be in contact with Julius. How hot is the 100 watt making the temp? I would try using the 80 watt 1st and see how that works and go from there. I don't think any watt less than 60 would help very much(to low heat). Also over head lights also kill humidity a lot, another option would be attaching another UTH to either the bottom or the side of the tank to help with the temps.