Hey bro, you shld b using low flowrate for the POX4, as long as the media is tumbling and not overflowing, it is ok.
You will noticed slower growth of diatom on glass for initial stage, i have topped another 50% of media according to tank capacity after 1st 2 weeks with no adverse effects. There maybe no signicant difference after tht as it will take some time to compensate for the large amount of phosphate in live rocks.