Have you already set the current fill level as reference value? If not, it can be only done in the menu of the device itself. Go to the main menu and navigate to sensors and select the fill level. There is an option available that sets the current fill level as 100%. Select this one and confirm it by swichtung to yes and press enter (center button). The calibration will take a few seonds.

It is also normal that the pump will slow down while doing this because this is the magic behind this feature. There is no real fill level sensor integrated into the aquaduct. We calculate the fill level based on a specific pump speed and time measurement. This is also the reason why you have to set an initial value manually - afterwards it works automatically.