Barley Bales have a long history of keeping ponds clear and free of excessive algae. Supposedly, long ago in Europe a Bale of Barley/Straw/Hay fell into a pond and stayed there for a few weeks.
When it was noticed it was apparent that surrounding the Bale the water was clear, a few inches/feet away from the Bale the water again became filled with algae. While the story may or may not be true, it does work. What happens is when Barley, Straw and many other types of grasses become dry they develop a small black fungus. This fungus is the real ‘miracle worker’ of this process. As it breaks down the Barley/Straw it releases a controlled amount of Hydrogen Peroxide which prohibits algae growth. Thus, it is beneficial to have this situated in a spot where it can float in or near flowing water; otherwise the benefit gained from the Bale will be very localized.