Common sense instead of high-tech. A magnet on the outside of the ceramic wall follows the movement of another magnet inside the tank, which in turn follows the water level. It's simple and effective, and will attract the curiosity of your guests.
If the outer magnet doesn't seem to be moving, it's probably on the wrong side: It must be able to roll on the ceramic and must therefore be placed on the rounded side (instead of flat side), facing the bottom tank wall.
If the magnet doesn't want to stay in this position, hold it and insist, the inside magnet will roll into the good position.