The sensor is not too special just a metal electrode... All electrode pickup signal with respect to supply ground. So electrode mounted on the over head tank should have ground at most bottom & low,high electrodes are placed at desired location...
The electrodes metal should be a corrosive, oxidation resistance material so that the sensitivity will not change. If you want you can adjust the sensitivity of individual electrodes by varying the potentiometer. all the electrodes are having individual led indications...
The circuit itself is a very old flipflop method & already available at internet. i have added a opamp comparator to adjust the sensitivity. I post this circuit here because of very compact PCB design & for hobby circuit builders to try...