the water at the bottom of a lake is always at 4'C ( if the lake is deep enough ) the more the water is cold the more heavy ( dense it is ) until it reach 4'C under than get a little bit less dense. But I will use a Pump , I don't want it run 24/7 it will cost to much in electricity, and over heating too, I can not throw a submersible pump in the lake, It can be too dangerous if someone cut the electric wire. I don't want also condensation, so I will adjust the flow for keep the tank may be at 20'c . don't know I will experiment, not to much info on that on the web