I recently set up my 75 gal reef tank. I have a 20gal sump. The sump is divided into 3 parts (I'll try my best to explain this). The overflow comes down into one compartment. It then overflows into another small compartment. The water flows from the bottom of this compartment into the last. My skimmer and return pump are in this last compartment. If this makes no sense I'll try to post a pic. I currently have bio-balls in the small center compartment but I'm gathering that those are a no-no for a reef tank. Would it be beneficial to take the bio-balls out and put some live rock in that part of the sump?? Also, I'm pretty clueless about the refugiums. Is there any way that I could use this small area for any type of a refugium?? Thanks again for everyone's help!!!