You simply must try this product - it's incredible! Crystal Cubes are simply placed into the Pool Skimmer Basket to give pools extra clarity and sparkle. Crystal Cube is a concentrated clarifying gel block which bonds sub-micron particles including algae. They are then large enough to be trapped by a filter. Use with Sand and Cartridge Filters to get DE quality water. 140 grams. The crystal cube lasts for one month. Place cube in the skimmer basket after ensuring chlorine and pH levels are correct. 24 hours after adding the Crystal Cube, backwash filter twice.