Reef Factory Smart Roller M Question

[claydogg84]

I assembled my roller yesterday and have a question regarding the adjustable flap on the side. The manual says the following -

"When the water drain level is stable, adjust the water level in the device by sliding the flap so that the water inside the device is at the "Water level"."

Is this just for initially setting up the roller before startup? I don't understand if I'm supposed to leave this flap partially open or closed. It's confusing me because I'm not sure why I would want the water from the sump to enter the roller. I tried to submit a support ticket to find out but the form needs the serial of the product which I didn't snap a picture of before leaving for work this morning.

 

[Reefing Madness]

 

[Snoopy 67]

Eventually I just closed the overflow port.
Then I raised the float assembly as high as it would go (touches) after drilling & tapping another hole
approx. the same distance between as the others.
No problems since.