You can shock and swim! Sometimes the water is cloudy, and the combined chlorine is 0, but the chlorine starts to get eaten up = this is because there is enough chlorine in the water to keep the algae from taking over, but not enough to completely kill it all, leaving you with cloudy water. The good news is that if you take it up to shock levels now, you won't have to continue at shock level because it should kill it all once and for all. If you wait, and the aglae takes hold, it will take a lot more effort to kill it - you will have to keep it at shock until it holds overnight. You can swim with shock levels of 20ppms, just wear an old bathing suit.