Thanks for the replies. Unfortunately, I don't have a way to put up any pics right now.

For the moment I'll go with the assumption the readings are accurate. I need to figure out a way to verify it without buying another thermometer.

The open part of the screen top IS on the cool side of the tank. I was thinking if I put it on the warm side, the heat would just go straight up & out. Should I reverse it? Maybe less heat would get to the cool side, but once it got there it would be trapped making it stabilize at a warmer temp?

I already expected to need to get a t-stat by the time winter gets here. It does get cold here, but not as cold as Denver! I just thought I was done with the spending for right now. Oh well. I guess I'll be heading out later to get a t-stat and another UTH. I assume a smaller size UTH is ok for the cool side? Or does it need to be even coverage, just kept cooler by the t-stat?

I do already have a very thin make-shift insulation on 3 sides of the tank. My plan is to get that foam poster board stuff and decorate it to make an attractive backdrop for the more permanent solution. I hoped with the mostly mild fall weather getting it done in a week or 2 would be soon enough, but maybe I should get some thicker cardboard so the temporary insulation will be more effective. Maybe I should also get something that could be put on the front of the tank at night to minimize night time drops in temp?

In the very short run, I've got a lamp I can turn on and aim where ever necessary for a few minutes at a time to raise the temp. It makes the temp jump up pretty fast, so I don't even step away, just watch the gauge. But once I've done this it drops off a lot slower.

Thanks again! I welcome any more suggestions or comments.