regulating the overflow?

[filly]

Ok guys, I stink at physics so maybe you can help out here. I have one overflow at the end of a 300 gallon tank. We were using both a 1' and 1 1/2' bulkheads to dump in the sump below and returning up over the back of the tank. We have a Dolphin Amp3600 for the return pump. My question is this. We used the durso's with plastic tubing coming out of them and valves to control the amount flowing in. If we take out the plastic tubing and just leave the holes, the water level won't stay high enough in the tank (below the hood). I had to cover the 1 1/4" durso an just have the 1 1/2" durso and now we either slowly fill or slowly drain. I can't find a happy medium. If we have both durso's used without the tubing, the tank remains constant but way below the level we need. HELP!!! Not sure what to do next to fix the problem. I have pictures here:

http://photobucket.com/albums/a117/filly_green/?sc=2

Thanks