The idea behind this design is to maximize fish bioload. The trickle filter is used to maximize gas exchange. The refugium is to maximize palnkton survival. The overflow feeds both the trickle filter and the airlift to the refugium. Some of the plankton will make it into the refugium and some of it will fall into the main display. The tank is 4'x2'x2' and the refugium is 8"x8"x48" flowing by Durso overflow through bulkheads into the main display
Dan I think it works against itself bud. The byproduct of the bacteria that inhabits the bioballs is nitrate, so you can count on nitrate laidened water. then it goes over the dsb. Now the bacteria that inhabits the top layer of the dsb are they same type of bacteria, the bacteria that will denitrify the water are below them. So you are asking that top layer bacteria to take it is own waste, I dont think that is going to be effective.
All your plumbing and so on looks great I think you need to take another look at the substraight you want to use. I think if you went with a finer particle bed (say a mud bed) you would have the anaerobc zone higher (almost on the surface) I think then the nitrification/denitrification system would e a hell of alot more effective.