Hey Jon ... I am drilling holes for a closed loop on my 300 right now actually ( tonite ! ... just taking a dinner break
)
I am drilling a 1 1/2" input ( to pump ) on each side and (2) 1" outputs ( back to tank ) also on each side ( total of (2) x 11/2" and (4) x 1" )
I will be reducing everything initially to 3/4" so that I can run my 2 Mag18's and as my circulation needs increase I can adjust accordingly
My return from my sump is a 4MDQX also but I found that too much back pressure will cause it to cavitate and produce microbubbles so it is pretty unrestricted and plumbed closer to the surface to keep the surface clear and produce a nice shimmer
Cheers