This past Spring I brought my pool's borate level up to 30 ppm and salt up to 1000 ppm.
I've noticed that the pool remains crystal clear even if the chlorine levels drop way below recommended.
Yesterday I tested and the chlorine was 0 ppm on the Chlorine Drop test. But the pool was crystal clear.
With a CYA level of 40 ppm the Chlorine / CYA Chart recommended chlorine level is about 5 ppm. So I dutifully added bleach.
In addition to algae, I suppose we have to worry about other microbes. So maybe we need the chlorine levels to kill them. (?)
But it is attractive from a cost standpoint to think that we may need to use next to nothing for chlorine. Along those lines, I'm down to running the pump and filter to 3 hours per day, and I think I could go lower. The back pressure remains low so I've not had to recharge with new diatomaceous earth, either.
Of course another factor is that we have a very light swimmer load (my wife and me, except a week with neice and nephew and parents). But that didn't keep the pool from getting cloudy occasionally last year with the same swimmer load!
So maybe the Chlorine / CYA Chart needs two more dimensions: borate and salt. (?)