Phi, the flow through the fuge should be slow to give the macro/sand time to process your water and "clean" it. But my personal experince has also shown that too slow of flow INSIDE the fuge will cause some problems like a lot of nuisance algae and detritus build up. I have a small amount of water going through mine but I also added a small powerhead inside to keep things moving, ya know? The MJ400 should be enough!