Skimmers take a few days of running before they start pulling the scum out of the water, also if your tank is new, there may not be much to pull out of the water. If you can rule those two things out, then you may need to adjust it (not familiar with the skimmer so I cannot tell you how). Most skimmers you can adjust how much air is coming in and how much water is coming in, you want a dry foam in you cup.