The contactless sensors have the advantage that they can be located outside the tank (they measure through the glass) and therefore are protected from soiling. Another advantage is the simple mounting. They are stuck from the outside to the glass, this way you avoid disturbing cables inside the aquarium. They are ideally suited for Auto Top Off and Leakage detection.

The disadvantage of this kind of sensor is that already a little amount of water is registrated as level, e.g. splash water which is running down the glass. Already a moist glass will activate the contactless sensor.
That is why the contactless sensor is not suited if the glass above the water surface is permanently moist or polluted. So the application in a skimmer is not possible.

The contactless sensor is also not suited for the automatic water change, since the glass remains wet above the water level. Therefore the sensor would detect a water level.