Hang-loose, FWIW when I have used the Phyco it takes a fair amount on a spot treatment basis to get rid of the algae, just as Bill had said. The effect is diluted, or you might say friendly, or forgiving. So, what Esshup says about the pharmacy product that is even more diluted would seem accurate. It probably would just disperse in the body of water so fast in wouldn't contact the algae enough. I also have a hard time understanding the "chemical" part so don't feel that you are alone. I have experienced that it works.