Clears green matter for a sparkling clear aquarium or pond.  Clear Water Block has a slow release delivery system that helps to condition the water and clear the unsightly green matter in your aquarium or pond.  When used strictly as directed, it is harmless to fish, plants and filter systems. Each block has 6 squares and each square treats 200 litres of water.  1 block of 6 squares treats 1,200 litres of water.