Formula “O” is a non-chlorine, blended powder that combines most of the typical supplemental water chemicals (i.e. shock, algaecide, clarifier, etc.) to make a once a week, easy treatment rather than using multiple, separate chemicals. This blend is registered with the EPA and will burn off additional contaminants left behind from shock, rains, winds and organic bather waste and is gentle enough so that you can swimin 15 minutes. With Formula "O", this non-chlorine shock has the ability to generate multiple levels of oxidation. Supplemented with our specific blend this product, works far better for water clarity, freshness, and safety in residential pools.