3 reasons...1 you need 2 pumps to run an external, which frankly is too much trouble for me. 2 now I can add something into my stand, as I have more room outside my sump... perhaps a fluidized reactor, or a bigger ca reactor, or a dsb, or turf algae....ooh the possibilities. 3, I'm a sucker for the latest and greatest, and have had good experience with reef octopus, so bought their new one.