Treating pond algae is not nearly as difficult as you may believe and does not require the use of potentially harmful chemicals. Algae are basically just phytoplankton that multiply very quickly within ponds and turn the water an unattractive tint of green. It's a notorious condition and is a problem that most, if not all, pond owners will eventually have to deal with. Fortunately, the organic method by which algae can be treated is also the easiest, safest and most effective.
Stuff the barley straw into the mesh bag and tie it shut using the rubber band.
Place the bag of straw either in the center of the pond or in the area with the most water flow. As the barley decomposes, it will release a chemical which eliminates algae.
Spread the water lilies over the pond, making sure to cover about two-thirds of the surface. You can also use duckweed or water hyacinth or a combination of all three. The plants will block sunlight and utilize nutrients that would otherwise be used by the algae to multiply.