I just use a floating swimming pool digital one tied on a string. Only works on the surface but it gives me a baseline to go by and I can throw it out all around the ponds edge or take it out by my boat to the middle of the pond. Other than aireation I really don't have any control over what the deeper water temp is anyway. On another note my pond is only 12 ft deep in the deep end.

Dear Alcohol, We had a deal where you would make me funnier, smarter, and a better dancer... I saw the video... We need to talk.