You do not mention the size of your drain. You can only put in as much as can drain out. On the CL system is your tank already drilled for this? A actual closed loop does not run through the refugium. It takes tank water and recirculates it back through the tank. I do agree with the amount you should get flowing through for a refugium. A sump may allow a little higher flow rate but not much. A refugium with plants in it will add more benifits, The plants produce oxygen when the light is on over them and they also will reduce nitrates. Plants also add CO2 when the lights are off.