I am setting up my aquarium again after being down for a few years. With the move I redesigned my plumbing and sump setup, and purchased a new return pump rated at a max flow of 3,645 gph. My overflow is setup as a dual herbie; drains are 1" bulkhead and 1" pipe. For a return off of the pump I am using a 1 1/4" return pipe that T's into 2 1" pipes, and each of those T into another 2 3/4" pipes over the back of the tank (4 3/4" returns in the tank total). I was expecting to dial the pump back quite a bit (maybe to 60-65%), but I am running it at dang near full power. My surprise is that my valves on each of my 1" drains are approximately only halfway open; so I am guessing I am probably only moving 800-900 gph with the pump at max power. My intent was to be turning over somewhere between 1500-1800gph in the tank, and also be able to direct some flow over to a manifold. Right now I have the manifold off. Is there something that I am missing or not considering that is impacting the amount of water I am moving? I can take pictures when I get home if that helps.