Barley straw can be decomposed in the water, as it decomposes, it produces and releases many compounds, one of which may actually control algae. The chemical compound does not take out expecting algae cells but it inhibits and prevents the growth of the new algae cell. So when the old algae cell will naturally decay, there would be no new sprout to colonize your pond. The population of the filamentous algae is therefore controlled. Actually, there are other kinds of straws that can be used in your ponds. You can use wheat linseed and oil seed, but as studies showed, barley straw is the most promising of all the straws available.
How much barley straw needed would base on the pond's surface area rather than its volume. Studies have shown that efficacy of the barley straw is highest at the amount of 0.025 pounds of straw for every square yard of the pond's surface area. If your pond is a little bit smaller, like an ornamental pond measuring four square yards, only 0.01 pounds of barley straw is needed. IF your pond is an acre wide, it would require as much as 107 pounds of barley straw or 2-3 standard bales. If the pond already has history of filamentous algal problem, more barley straw is needed. Try using 225 pounds per surface area to ensure the efficacy of the barley straw.
The decomposition process of the bear lye straw and the production of the needed compound mainly rely on the presence of oxygenated water. If your pond is a little bit small, you can just place the small amount of barley straw, preferably loose in a mesh bag, and then you can place it directly in the water. You can add some weight to make the mesh heavier so in can sink on the pond bottom.
If you have a bigger pond, you need to exert more effort. Each bale should be loosen up as much as you can so that it will immediately decompose in the presence of oxygen in the water. Same goes with the process, the bale should be put in a permeable mesh bag, and these bags should be placed around the perimeter of the pond in water. Make sure that the bag is not placed deeper than 6 feet to ensure the efficacy because most of the algal production occurs in shallow water. Replace the bales inside the mesh as needed often. If applied properly, the growth spurt of the filamentous algae can be prevented.