A customizable multi-stage water treatment system comprising: a base cartridge (14, 94. 156) having a first end, a second end (26, 120), and a sidewall (27, 108) extending between the first and second ends and surrounding a base interior volume, wherein a base treatment medium is spaced within the base interior volume; a first variable cartridge (16, 158) configured to be removably engaged by hand with the base cartridge without the use of tools, wherein the first variable cartridge includes a first coupler end engaged with the second end of the base cartridge, a first receiver end (38, 122), and a first wall (39, 110) extending between the first coupler and first receiver ends and surrounding a first interior volume, and wherein a first treatment medium is spaced within the first interior volume; an second variable cartridge (62, 9, 160) configured to be removably engaged by hand with the first variable cartridge (16, 96, 158) without the use of tools, wherein the second variable cartridge includes a second coupler end engaged with the first receiver end (38, 112) of the first variable cartridge, a second receiver end (66, 106), and a second wall (68, 112) extending between the second coupler and second receiver ends (64, 66) and surrounding a second interior volume, wherein a second treatment medium is spaced within the second interior volume; a connector (12, 92, 154) coupled with the first end of the base cartridge (14, 94, 156) and having an inlet for receiving an intake water from a water source and an outlet for dispensing a treated water to a user, wherein the connector is devoid of treatment medium, and wherein the base (14, 94, 156), first variable (16, 96, 158), and second variable cartridges (62, 98) together are configured to receive the intake water, to bring the intake water into operative contact with the base, first, and second treatment mediums, and to change the intake water to the treated water; and a keyed mating arrangement ( 152, 168, 176, 186, 196) that is disposed on the connector (12, 92), the base cartridge, first variable cartridge, and second variable cartridge and is configured to prohibit the base cartridge (14, 94) from matingly engaging in a fluid tight manner with the second variable cartridge and prohibit the connector from matingly engaging in a fluid tight manner with the first and second variable cartridges (62, 98). a base cartridge (14, 94. 156) having a first end, a second end (26, 120), and a sidewall (27, 108) extending between the first and second ends and surrounding a base interior volume, wherein a base treatment medium is spaced within the base interior volume; a first variable cartridge (16, 158) configured to be removably engaged by hand with the base cartridge without the use of tools, wherein the first variable cartridge includes a first coupler end engaged with the second end of the base cartridge, a first receiver end (38, 122), and a first wall (39, 110) extending between the first coupler and first receiver ends and surrounding a first interior volume, and wherein a first treatment medium is spaced within the first interior volume; an second variable cartridge (62, 9, 160) configured to be removably engaged by hand with the first variable cartridge (16, 96, 158) without the use of tools, wherein the second variable cartridge includes a second coupler end engaged with the first receiver end (38, 112) of the first variable cartridge, a second receiver end (66, 106), and a second wall (68, 112) extending between the second coupler and second receiver ends (64, 66) and surrounding a second interior volume, wherein a second treatment medium is spaced within the second interior volume; a connector (12, 92, 154) coupled with the first end of the base cartridge (14, 94, 156) and having an inlet for receiving an intake water from a water source and an outlet for dispensing a treated water to a user, wherein the connector is devoid of treatment medium, and wherein the base (14, 94, 156), first variable (16, 96, 158), and second variable cartridges (62, 98) together are configured to receive the intake water, to bring the intake water into operative contact with the base, first, and second treatment mediums, and to change the intake water to the treated water; and a keyed mating arrangement ( 152, 168, 176, 186, 196) that is disposed on the connector (12, 92), the base cartridge, first variable cartridge, and second variable cartridge and is configured to prohibit the base cartridge (14, 94) from matingly engaging in a fluid tight manner with the second variable cartridge and prohibit the connector from matingly engaging in a fluid tight manner with the first and second variable cartridges (62, 98).