The 42MM Filter Cap by HydraPak threads seamlessly onto all sizes of HydraPak's 42MM threaded Seeker water storage bags and Flux flexible bottles morphing them into filtration solutions. The end result being lightweight filtration kits that can either be used to squeeze filtered potable water into other camp bottles and containers or to drink freshly filtered water from a Seeker or Flux. Both deliver fast and easy water treatment around camp or on the go. Filter water on the go. HYDRAPAK 42MM FILTER CAP