Pond dyes or pond water sunblockers work by blocking the sunlight rays that penetrate through the water. This is achieved by a special formula of using pond dyes and chemicals. These dyes discolor the water (and your hands, clothing, etc. if exposed.) These products are used because microscopic plantonic algae which turns your pond water green, needs sunlight (and nutrients) to grow. Submersed pond weeds need the same. When you use a pond dye, you are therefore 'starving' them of some of the necessary sunlight they need to grow, thereby reducing their growth or eliminating it.