what controls water heigth

You have your sump setup the right way. The proflex sump has 3 sections first section is the filter socks the next section is the fuge or whatever you want to put in there. Some people just drop extra live rock in there. The final section is for the skimmer and the return pump.

To adjust the size of the back section there is a plastic piece that slides up and down. There is no way to lock this piece in place as it uses water pressure to stay up. So all you have to do is reach in the sump and lift that slider piece up to the desired height you would like it at and keep holding it up until the water flows over it. One this happens the pressure should be high enough and it will hold the flap up. A lot of people use epoxy to secure the flap 100%.

This will probably cause the water in the 3rd section to drop even lower. Next what you need to do is fill the tank with more saltwater until it reaches the required height that you're looking for.

It takes a little tinkering to get the proflex setup correctly, but when it is man is it nice.

 

That's definitely a versatile sump design. I found a little bit of read up for it, although I'm not sure which style you have. Hope this helps, and thanks for the correction Monroe. Proflex Instruction PDF

 

Do you have an ATO? It sounds like your return chamber is where your going to set your skimmer? Your going to want that to say as consistent as possible so you don't have to consistently adjust your skimmer. That would drive me crazy.