It sounds like your temps and humidity are just fine, other than moving the hotspot back down to 90 seems like everything is fine. I don't really think the environment is responsible, what kind of substrate do you use?



The only time I have seen a ceramic lamp used on a tub was when a friend of mine tried it...

He cut about half the lid open so that there was plenty of space all around the heating element and covered it with screen. Then he used wire to lift the ceramic fixture... I think he had it in a deep dome... off of the screen. The idea was the heat would be directed downward as much as possible without heat being passed into the plastic. It kind of worked but then his humidity took a huge hit because half his lid was missing...

I might suggest that instead of going from the top you go from bellow, since heat rises anyway. Try getting another UTH and putting it under the cool side so that you dont create another hotspot but rather heat the tub.

All this being said your set up sounds fine the way it is. Your time might be better served looking into other causes. Maybe the temps aren't stable? Maybe there is a night-time dip?