Rx Clear™ Filter-Renu: Granulated compound cleaning agent for sand, cartridge and DE (diatomaceous earth) filters. Add through skimmer and allow to remain in filter 30 to 40 minutes before backwashing to purge out dirt, oils, minerals and organic matter from filter systems. Add once a month to remove scale and rust. Use 1 lb per every 8 sq. ft. of filter surface area.