Agreed with blumoon, interesting and helpful thread we have here. I Use bioballs for my 120 FOLWR but not for my reef. The reason for their use in the FOWLR is because i dont believe i have adequate quantities of LR (although i have an estimated 100-120 lb in there) to keep pace with the first steps of the filtration process Steve lays out above. nor do i use my sand bed as any type of filtration as i regularly vacuum it because i feed fairly liberally with all the aggressive eater i have in there. for these reasons i feel the bioballs are needed to lighten the load placed on my LR in the DT. so far no issues with the FOWLR after 2 years of use but i perform monthly cleanings on 1/3-1/2 the bio balls to insure proper filtration accompanied with 30% wc every 2 weeks. In my 110 reef tank where i have a much lighter bio load and a sand bed that goes un vacuumed to allow for some bacteria to be established and with 120 lb of LR i dont see any need for bio balls. my skimmer along with my LR has kept my nitrates at unreadably low levels so i dont see any reason for having the added bio balls to this system. So it makes some sense to me why people with FOWLR would opt for bio balls as these tanks are typically heavily stocked with large fish that are messy and produce more waste then fish found in reef tanks. the added bio balls may just be the needed backup to help keep the heavily stocked tanks in check where a fuge is not in use.