i had the same question... i just bought the NW 200... i set it up in the bathtub to clean it before i used it....you can kind of tell how it will do once you see the water running through it......when you extend the water level tube, it skims differently.... i think it skims wetter the more you extend the tube.... (i think)....i am going to set mine on the bottom of the sump..it will have about 13 inches of water....i will have to extend the tube about 2" for this...the tube w/out being extended on mine is 11" high, so if i wanted the tube to be all the way down and not under water , i would have to build a 2" stand.... try setting it up in the bathtub and i think it will help you see how it works!!!