I think what needs to be considered here is that water "takes the path of least resistance". Is there any guarantee that 1/2 the output water from the Mag 12 will make the right hand turn at he T 18" away from the sump? Ideally you'd want to push the return water the same distance, with the same number of turns. You may have to experiment with valves inside your tank on the spray bars or between them. That way you can dial down the final water volume on the top (fastest flowing ) bar and move the volume to one of the others.
I hope some of this makes sense. I tinkered and tinkered with my setup. I am using a MAG 18 as the return and I have an inline ball valve and only one spray bar, and it has worked with great success for 3 years now. I have two maxijets in the tank (pointing at each other) that give great circulation and prevents much of the detritus from settling on my rock, corals, or sandbed.
Good luck!
I hope some of this makes sense. I tinkered and tinkered with my setup. I am using a MAG 18 as the return and I have an inline ball valve and only one spray bar, and it has worked with great success for 3 years now. I have two maxijets in the tank (pointing at each other) that give great circulation and prevents much of the detritus from settling on my rock, corals, or sandbed.
Good luck!