Value kit with money off the usual price of 6 service kitsr the filter cartridge works like a waste bin trapping dirt once full it should be thrown away and replaced with a new oner on average this should be carried out every 6 to 8 weeks but this is dependant on the size of your biorb and how many or the types of fish it is stocked withr your biorb instruction manual will also always inform you of the best time to carry out a filter changer a partial water change of 30 should be carried ou