Barley Straw Pouches – Natural and effective against green water algae. Use one single pouch in ponds up to 800 gallons (3500 litres or 130 cubic feet) of water. Harmless to pets, wildlife and pond plants. Natural decomposition by micro-organisms gives rise to anti-algal factors. Single for 800 gallons every 6 months. Twin for 800 gallons every 12 months or 1600 gallons for ½ year.