This filter was discontinued by MARS Fishcare in 2012. No 4-sided cartridges have been made for this filter for at least two years. You can use other API bagged media in place of the cartridges. 108A (Bio-Chem Zorb) is the most popular. I do recommend cutting a small piece of foam to place in the bottom. API used to manufacture this for use in the filter but it is no longer available. Keeps stray particles from going through the system. Also grows bacteria which is good for biological filtration. I recommend item 723A used in the Filstar but could be easily cut to fit the Nexx.