All of that baqua-goo has gone through your sand, and most has been backwashed away. The sand, however, being the rough stuff that it is (which makes it good for filtering) will hang on to bits of the baqua-goo. Your chlorine will spend all its time trying to sanitize the goo left in your sand, and there will be none left to sanitize your water, which sets you up for all kinds of trouble - pathogens, algae, oh my! The easiest (and probably cheapest) way to avoid all of this is by changing your sand and cleaning out your filter!