Sue will be able to size you a system.A single 9 inch diffuser is just that, it would work poorly compared to the effect of having two 9 inch discs spaced on a single weighted station, thus the term CoActive. The distance between the two discs is dictated by your water depth. The deeper the further apart they are.At some depths additional discs are added to a similar weight base. Vertex has an online electrical calculator where you plug in your cost per KW on a given system and the cost is displayed http://www.vertexwaterfeatures.com additional information on diffusers can be found at http://www.cleanponds.com in the "A word about diffusers " section off of the homepage.