you have to be very careful using filter media and bio-balls in marines, as if they are left for a while and get clogged up then they do a lot more harm than good. Ultimatly if you want a reef tank, you will need live rock and not the bio-balls. Once you have gone live rock, the external filter will be a nice place to run phosphate remover and other bits and bobs. Live rock will give you far more efficient filteration than bio-balls ever will