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Chemical interactions in your pool are complex as shown below. WaterGuru SENSE along with advice in the WaterGuru app can help.
97% to 99% of the chlorine is bound to cyanuric acid (CYA) under typical pool conditions. Fortunately it only takes a small amount of unbound active chlorine (hypochlorus acid) to disinfect and prevent algae growth. The active chlorine level is proportional to the FC / CYA ratio. This means that you should raise your free chlorine target as the CYA level rises.
For every 10 ppm free chlorine (FC) added by trichlor, it also increases cyanuric acid (CYA) by 6 ppm. At typical 1.5 ppm FC per day chlorine usage, this represents an increase in CYA of 27 ppm per month.
Lowering phosphate levels does not kill algae; it slows down their rate of growth. With lower phosphates, one can control algae growth with a lower active chlorine level.