Know Your Algae
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Most chlorine is bound to cyanuric acid (aka stabilizer) and it is the unbound active chlorine that kills/prevents algae and provides disinfection.
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.
Stabilized chlorine products, such as Trichlor tabs and pucks, increase the cyanuric acid level over time thereby lowering the active chlorine level so algae can then grow faster than it is killed.
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.
Phosphates are an essential nutrient for algae growth so keeping phosphates low helps prevent algae.
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.