A customizable multi-stage fluid treatment assembly typically includes a connector, a plurality of cartridges, and a cap. The plurality of cartridges have a treatment medium spaced within an interior volume of the individual cartridges, between the ends thereof. The ends of the plurality of cartridges are configured to receive a fluid, bring the fluid into operative contact with the treatment medium, and dispense the fluid from the opposing end of the cartridge. The connector is coupled with one end of the plurality of cartridges and has an inlet and an outlet for receiving and dispensing the fluid to and from an appliance. The cap is coupled with the other end of the plurality of cartridges, enclosing the fluid treatment assembly, which is configured to be received in a cavity of an appliance. The cartridges of the plurality of cartridges may be individually replaced with cartridges to meet customized needs.