Abstract: Embodiments of a filter cartridge for a sports bottle are shown and described. Each embodiment has a member for containing or attaching to a filter media and having a means of securing the cartridge inside the bottle neck without requiring modification of the bottle or bottle cap. The preferred cartridge includes a generally cylindrical cup or ring for holding media, and an extension that fits between the cap and the bottle neck to secure the cartridge in place while preserving a water-tight seal. The extension has a slanted inner surface that diverts the cap's knife edge inward slightly and then seals with the outside surface of the knife blade. As the cap is installed and the knife presses down against the extension, the knife forces the expandable or flexible extension outward slightly to seal against the neck.