Mark is correct about lower pH being more effective for chlorine, but in the presence of CYA the influence on chlorine effectiveness vs. pH is significantly reduced. These graphs show how a change from a pH of 7.8 to a pH of 7.2 improves chlorine effectiveness by less than 30% so increasing FC by 30% would do the same thing. If there were no CYA present, then the same 7.8 to 7.2 pH change would improve chlorine's effectiveness by over 110% (more than double).

Richard