I imagine the water dish is right under the CHE?
I would tell you to move the CHE, but if you are just getting temps dialed in, probably not the best idea.
Also, not much room to move stuff around with a 20G.
You can a) put in a smaller water dish and move away from the CHE, B) uncover part of the top of the tank (assuming you covered with foil as you said were going to), or A and B.