Provided are media cartridges that have adjustable bypasses, and systems and methods that use them. Aspects include an adjustable bypass filter cartridge where a treated stream and an adjusted non-treated stream combine prior to post-filtration and the percentage or ratio of treated the and the non-treated water is adjustable at the time of installation from the outside of the filter cartridge with a known or predictable level of bypass. Media cartridges comprise: an inlet in fluid communication with a feed chamber and an outlet in fluid communication with a mixing compartment; a media compartment containing a media; a bypass port in fluid communication with the feed chamber and the mixing compartment. Such cartridges can be adjusted to meet the needs of a wide variety of end uses.