After Shock is a chlorine reducer. After Shock can also be used to quickly lower chlorine (about 30mins) An 16oz. jar of After Shock will lower the chlorine level 4 ppm (parts per million) in 15,000 gallons of water! It’s important NOT to overdose with this product… it will actually stay in the water until it has chlorine to neutralize, which means it could be very difficult to reestablish a chlorine residual if you overdose! 16 oz container.