Hi Andreas - I just had a thought - you are going to run a TS4 ON A 6FT TANK!!!! HOLY COW! And I thought Puppy had a HUGE ARSED SKIMMER on his 1.5 metre! HAving a TS4 on a 6ft is like putting a bloody V12 engine in a little beetle!!!!
On your your question - it's the riser tube where the water exits the skimmer. You move this tube upwards to skimmer wetter, and downwards to skim drier...
The water level INSIDE the skimmer, rises when you move the pipe upwards, when you move the pipe downwards, the water level goes down inside the skimmer....
At what water level is your sump water, in the chamber where you have skimmer?