A water treatment system, such as a water softening system, having a water treatment tank with at least one distributor plate mounted inside to support filter media and/or ion exchange resin. The water treatment system is designed to treat hard water with a packed ion-exchange filter media and has a distributor plate design for facilitating the ion-exchange within a water softener resin bed, as well as facilitating the regeneration of the resin bed. The distributor plate presents cavities to the topside for entrapping filter media, and the cavities have narrow slits located at the base for allowing fluid to pass. A method for assembling the water treatment tank and supporting inserted distributor plate is shown. The distributor plate rest on and is supported by a domed-shaped structure that can be placed in the bottom portion of the water treatment vessel.