Sustain® Summer Shield® W8004182 1 qt Chlorine Extender 12 per Case provides water with an extra level of protection against algae. It does this by creating a reserve, or backup, of chlorine fighting power that is released on-demand when routine sanitizing levels drop (below 1ppm), such as after a rainstorm or pool party. Sustain Summer Shield chlorine extender also contains a clarifier to gather suspended particles that can cloud your water, making them easier to be removed by the pool filter.