How can I adjust the water level in a Grohe inner wall toilet?
The geometry of the toilet bowl itself determines the water level, so it’s not adjustable. Try adding water with a bucket and you will see the surface area return to where it was designed to be. The flush action is designed to get as much force out of the six liters of water as possible, so there will be turbulence. I recommend closing the lid before flushing. With European toilets, it seems to be a matter of us adapting to them.