I like the surface skimmer box. It will help with removing the surface film that you sometimes see. While my tank was down, and water wasn't going into the overflow box (mine goes across the entire length of the tank), I could see the film form on the top of the water. My circulation helped to keep it agitated up there, but it wasn't the same. As far as putting carbon in the prefilter box, I think you can do it (the box is baffled...one section for the pump?), but I don't know if I would. IMO, the dirtiest water should make its way to the skimmer, in order to do the best job it can. But, if you don't have any other options, then it might work.