The perfect pre-filter for HOT Magnum media container and micron cartridge. Each Rite-Size Foam Sleeve is specially designed to prolong the life of filter media by reducing the accumulation of waste and dirt particles which can prematurely clog a filter. Fine mesh foam traps and holds dirt particles while the sleeve's open-cell construction promotes water flow--without impairing overall filter performance. For best results, rinse or replace monthly.