1. A customizable multi-stage fluid treatment assembly for an appliance comprising: a base cartridge having a first end, a second end, and a sidewall extending between the first and second ends enclosing an interior volume, wherein a treatment medium is spaced within the interior volume between the first and second ends, such that the first and second ends of the base cartridge 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 base cartridge; a connector coupled with the first end of the base cartridge and having an inlet and an outlet for receiving and dispensing the fluid to and from the appliance and configured to receive and dispense the fluid to and from the base cartridge, wherein the connector is configured to receive untreated water through the inlet and dispense treated water through the outlet after it has been treated by at least the treatment medium; a first variable cartridge having a first coupler end configured to be removably engaged by hand with the second end of the base cartridge, a first receiver end, and a first wall extending between the first coupler end and the first receiver end enclosing a first inside volume, wherein a first filter medium is spaced within the first inside volume between the first coupler end and first receiver end, such that the first coupler end and first receiver end of the first variable cartridge are configured to receive the fluid, bring the fluid into operative contact with the first filter medium, and dispense the fluid from the first coupler end of the first variable cartridge; a cap coupled with the first receiver end of the first variable cartridge and enclosing the first receiver end such that a sealed housing is formed with the sidewall of the base cartridge and the first wall of the first variable cartridge, wherein the sealed housing is configured to be removably engaged with a filter head assembly of the appliance; and a channel formed within the sealing housing between a select one of the inlet or outlet of the connector and the cap, wherein the cap is configured to direct the fluid to flow between the first variable cartridge and the channel, and wherein the channel is configured to allow the fluid to travel in a closed circuit to and from the connector through the channel, the first filter medium, and the treatment medium.