Crystal Plex Copper Sulfate Pond Algae Control Liquid (4 Gallons) Copper Sulfate Pentahydrate - 19.8% How do I apply Crystal Plex pond algaecide? &nbsp; The best way to apply liquid copper sulfate is For use in Ponds and lakes. NOT for use in Koi or Trout ponds. Controls many types of algae like filamentous, chara, and placktonic