i also have a g3, is a great skimmer , you wont be disappointed. the tube has to be above water so it can draw air into the skimmer. if you put it under water there wont be any air mixture in the chamber. try to put your thumb over the opening of the tube and you know what i mean. good luck with your new toy
oh BTW water level should be atlease 6" mines is around 8" and it works great