A: The 4-sided cartridge has been discontinued by the manufacturer, Mars Fishcare. The two filters that used this 4-sided cartridge, RENA SmartFilter and the RENA/API Nexx filter have both been discontinued for over 8 years.
There is *no* filter alternative for the Smart Filter. However, the Nexx Filter has always been able to use all API filter media that comes in pouches. This includes Bio Chem Zorb. You must remove the cartridge holder to use the pouch but it does work well. When these filters were originally offered in Europe they used pouches and not cartridges.
I also recommend getting the foam inserts to put on the bottom of the cartridge well. We have a few left but these have been discontinued also so in the future you would have to cut your filter material for bottom. Not required but it does keep small particles from circulating into the aquarium and does provide some additional biological filtration.