Blagdon Extract of barley straw is a totally natural means of resolving algae problems in garden ponds. Its unique formulation clears ponds of both green water algae and filamentous blanketweed.May be used in ponds with filters and all species of pond fish, plants and water using wildlife. Contains no synthetic chemicals or artificial pesticides. Aqueous concentrate containing barley straw extract.250 ml easy dose bottle. For initial dose 25 ml treats 909.2 litres (200 gals).500ml treats 2,000 gallons for 6 weeks1 Litre treats 4,000 gallons for 6 weeks