Hi, Ive got a fluval U4. I assume they are similar, they look it. Have you tried adjusting the flow controls to get the flow where you want it to go? I've put mine on the side wall and the top jet shoots down the middle of the tank - I find it is very strong at first and then starts to slow down as the sponge collects. Also can you see anything getting sucked through the sides of the filter on the intake or had a quick look at the sponges to see if there is anything getting collected?