Then why the secrecy?this product I have in mind already exists
Already mentioned that. Also asked the TS about their skill level for building electronic circuits, but I guess he must have gotten lost on other points within this thread.the circuit need be little more than a waterproof thermistor probe, a potentiometer, a comparator with little hysteresis, and a switching device
Sounds backwards, unless I'm missing something you're trying to convey.When the resistance is low, the fan should go off and the fan should run when the resistance is high.