I'm using SeaChem Matrix in my sump. It's recommended to use 1L per 100gal, and for my 65 gal tank w/ 20gal sump I have 2L, also with a filter sock. Pretty easy to find at any LFS too. I'm sure the sintered glass "bullets" would work tho. Be wary of any media that claims to "reduce nitrate" (even Matrix states this), the best way is still water changes.