Adding CYA reduces the percentage of chlorine that is active at any given time. That's bad, if you try to follow the 1 - 3 ppm recommended levels . . . but we don't!

In fact your target chlorine level should be 5 - 10% of your CYA level. If you operate that way, you'll have a fairly constant amount of ACTIVE chlorine present.

BUT, there's a huge advantage to higher levels of CYA *if* you follow our cl-cya chart: http://pool9.net/cl-cya/ . You can have FAR more chlorine in reserve, ready to almost instantly replace the ACTIVE chlorine, if it gets used up.