Crystal Plex is an algae control for ponds and lakes. Sold at Steinhauser’s, this liquid copper sulfate solution is designed to kill and control algae and some aquatic plants. Crystal Plex is 100% safe for humans, fish, livestock, pets, birds, swimming, consumption, and irrigation systems. However, it should not be used in Koi or Trout ponds. Keep your pond or lake environment healthy with Crystal Plex.