My local pool supplies store ran a free clinic on pool maintenance which was given by no other but a chem supplies rep. He went through the usual routine of "stabilized chlorine" at 1-2 ppm and weekly shocking at 5. According to him weekly shocking is needed because supposedly the bacteria become immune to chlorine. I thought they get immune just to antibiotics and I could have asked him how come it doesn't become immune to 5 ppm of shocking , but obviously there was no use debating him - they would have thrown me out