A filter cartridge containing resin beads for purifying fluids such as water and chemicals has a cylindrical housing with an interior compartment containing a fixed septum and a moveable piston-like septum. The resin beads are packed between the fixed septum and the moveable septum. The moveable septum is displaced upstream to account for expansion of the resin beads while the flow of the fluid through the filter cartridge causes the upstream septum to maintain a packing pressure on the resin beads. The downstream fixed septum will preferably have a cone-like shape.