There are many moments where shocking your pool water is a good idea. Not only should you shock your pool water before closing it for the winter, but you should also shock it at the beginning of the pool season. Shocking your pool water will kill any bacteria and algae that formed while it was closed for winter.  Shocking your pool basically means adding enough chlorine to raise the free chlorine level in the water. When shocking a pool, the free chlorine level should be about 10 times the combined chlorine level. The process of shocking a pool destroys all contaminants, such as algae and bacteria.