platapus,
On the top of your tank, that big pipe is your only return? And it goes to those 3 90 degree elbows, right? Were you planning on having any surface agitation? That's one thing I did not account for when I built my prop system. Unfortunately, it was an easy fix, I just modified some plubming in one tank and put a few powerheads in the other.
I think it's one of those things that a lot of people don't think of. A great way to push a lot of water around is to make you own nozzle with a 3" peice of PVC. The SCH-40 works the best. Take a heat gun or boil some water. You get one end heated up and pinch a little bit with your fingers. And then curve it just a tad. It will fit perfect back in any fitting and does wonders for giving the water a little bit of pressure at the tip and for surface movement.