I have 2 325's with a 3600gph Reeflo Dart. I probably set them up backwards so no pressure registers. I have dual overflows on a 180g piped together into the 1st OC 325 with scrubbies, lava rocks, and bio balls. To the 2nd OC 325 with the pleated filter and more bio balls in the center. Out to the pump and back to the tank split into 4 fan inlets.