An algaecide or algicide's a substance used for killing and preventing the growth of algae. Synthetic algicides Copper sulphate:It's very toxic. Natural algicides Barley straw, in England,'s placed in mesh bags and floated in fish ponds or water gardens to help reduce algal growth without harming pond plants and animals. Barley straw hasn't been approved by the EPA for use as a pesticide and its effectiveness as an algaecide in ponds has produced mixed results during university testing in the U… (More on Algaecide)