I just added one of these to my freshwater setup - I was getting too much debris/film on the top of the water.
It works pretty well, but is a bit noisy for my taste, and is touchy to adjust. The trick is that there is a set of inlets at the bottom, and a rod that sticks out of the top. Turning this rod adjusts how much water the inlets allow in. As these inlets are restricted, they try to pull water from the top. With a panted tank, the inlets and the skimmer are getting clogged more often than I like, causing air to enter the cannister filter. Not enough to break the siphon, but enough to make things noisy.
On a saltwater tank, you shouldn't have the clogging that I see.