I definately wouldn't use bio balls. They're ok for the inital few months of cycling, but eventually they begin to effect your nitrate concentration. In my opinon its best to go with a refugium rubble. Two questions, what type of corals do you plan on housing and what is the size of your tank? Im assuming your tank is atleast in the 55 gal or over range if you're using a 20 gal sump. 300 gph seems a little inadequate, because most overflow boxes for larger aquariums are rated for atleast 700 gph. You have to match your overflow rating with your return pump rating to the amount of water turnover for the type of corals you wish to keep. The last thing you want is a flood in your house because the sump or the tank overflowed. To answer you question vividaquariums.com has some pretty cool filter specs posted on their site.