A cartridge system for converting a bulk feeder which delivers a single water treatment material at a high rate to a bulk feeder that delivers two or more water treatment materials at a lower rate through locating a set of dispensing cartridges in the chamber of the bulk feeder with the dispensing cartridges held in position to maintain the fluid outlet port of one of the cartridges in fluid alignment with the fluid inlet port of another cartridge to provide cartridge to cartridge flow without having to circulate the fluid through the chamber of the bulk feeder.