or it could just be i dont no, the skimmer came with the overflow box, i thought it kept the film off the top of the water, but if i dont need it i would rather not have it in, i see film rather its on there or not, i just thought sence it came with it it must work better with it on, i could take it off, are there two differnt kinds of overflow boxes, im just talking about the black one that you can see hanging on the skimmer, i put it in the tank under the skimmer & tighting the screw to hold it on,