Your tank seems quite similar to mine (120 gallon, 36" tall stand - crazy plumbing
). I did 1.5" intakes to my Amp 3000 for closed loop, as the pump requires 1.5" plumbing in and out of it. The output bulkheads in the tank we did 3/4" because that is the largest size locline. What we did was reduce the 1.5" plumbing to 3/4" at the bulkhead.
Here is a link to my chronicle thread.....the first post lost a picture for some reason, but the link is there.....if you go down to the second post there is a link to my plumbing diagram and subsequent building:
NaH2O's Mini-Beast.
Here are a couple of pics of how the plumbing looks:
(the motorized ball valve was removed in this pic)
Plumbing on the inside:
