Abstract: A modular water conditioning apparatus for filtering or adding substances to water or similar fluids has a filter assembly with a hollow cylindrical casing extending perpendicularly upwards from a cylindrical base section. The base section has coaxial inlet and outlet tubes disposed perpendicularly outwards from opposite cylindrical walls of the base section. Within the base section, a hollow cylindrical boss, coupled to the outlet pipe, extends upwards. Cylindrical filter elements contained within the casing of the filter assembly are held in position and forced into sealing contact with the outlet boss by a cup spring pressed down on the upper surface of the filter element by a cap screwed down on the upper open end of the filter casing. Filter assemblies can be cascaded by inserting an outlet tube into the larger diameter inlet tube of an adjacent filter assembly.