Just to add to this, don't rely on check valves. They foul and don't work well. Bagillions of hobbyists have tried and they just kind of don't work unless you clean them weekly. Don't do it!
Hello Richard, I love your setup. That tank is exactly what I've been dreaming of and planning for . . . and thank you so much for the infomative guide to equipment and species etc. I'm so excited to have a guideline to use now. Thank you again! Paula
Other than the drain down problems that you have been talking about. You are still going to need more flow for a 125 gallon reef tank, than 1800 gph. Your plumbing if you can make it work will be nice because it will give a good amount of diffused flow with the 3 out lets per side, but you will want much more flow than that total in your aquarium. What I would suggest you do is go with a much smaller return pump, say 800 to 1000 gph, and use your 1800 gph pump as a closed loop, then you can plumb just like you have been talking and not have to worry about it draining back in the event of a pump failure.