Overflow and Standpipe Help

Hi guys i am setting up a 110 gallon with a corner overflow.
My question is regarding the stand pipes. Mine came with a Durso like standpipe for both the intake and return that is height adjustable.

I have a few questions:
Can someone please tell me why they are adjustable for height.
Second question is what keeps my display tank water level constant? Meaning why when water evaporates from it does it get replenished from the sump?
Last question is, how do i adjust the water level in the display tank. Do i toggle the ball valves on the intake or on the pumps return?
Thank ahead of time
Kevin

 

They make them adjustable so they can sell one product for most overflows. You don't want the water to fall to far because it will make a lot of noise, so set the height to keep water in the overflow just a few inches below the top.

Answered in your other thread.

With built in overflows your really can't. The water level is always going to be at the point where water flows into them and down to the sump.

When you get it set up, it will all be very suddenly clear to you

 

+1 to all the above.

Evaporation will lower the sump water level.. Like mike said.. the overflow determines the water level in the tank.