I find that surface extracters do take away from the mechanical filtration, I had one on a fluval 404 befor and didnt like it, but it did keep the surface clean. I dont think it will make the filter function any different or take away from its GPH, but im not sure as i havent done it to my fx5. i think the best way to keep the surface clean and not take anything away from the filter is to either put a spray bar on it, or point the outputs towards the waters surface. just curious, why do you want the surface extracter? do you have a panted tank?
