CYA is a stabilizer additive to Triclor chlorine pucks and is released into the water as the puck dissolves. CYA is short for cyanuric acid. In water content concentration of up to 70 parts per million, it helps chlorine effectiveness, but over 70 ppm, the opposite is true. The amount of time to kill bacteria lenghtens as the concentration of CYA increases. The ideal is 40 ppm.
Most pools have an effective algaecide present in the water to reduce the amount of chlorine needed for a clean and clear pool. Some pool owners use only chlorine and no algaecide and are satisfied with the results because the CYA levels are kept in check because the free chlorine is killing both algae and bacteria. It takes a lot of chlorine to achieve satisfactory results.
The danger of CYA buildup is loss of efficacy of the free chlorine needed to fight both algae and bacteria. If you are using triclor for disinfection ONLY , and have an algaecide in place, then CYA will build up quickly because for each 10 ppm of free chlorine produced, it also produces 6.0 ppm of CYA. Your CYA may be increasing by 6-8 ppm per week or 25-30 ppm per month.
However, most balanced pools contain an algaecide. If you don’t raise the free chlorine level correspondingly, algae will begin growing. This is also the reason that during the summer a pool starts out with no algae and eventually it has algae. What is happening is in the beginning you may have 30 ppm of CYA and keeping chlorine at 2.0-4.0 ppm. This is okay because 7.5% of 30 ppm means you need 2.25 ppm of free chlorine.
In a month the CYA goes up by 30 ppm. Now you have 60 ppm CYA and you need 7.5% so you now need 60 × 0.075 = 4.5 ppm free chlorine. In another couple of weeks, the CYA goes up by 15 and now you have 75 ppm CYA which means you need 5.6 ppm free chlorine. So the recommended level of 2.0- 4.0 ppm (and the EPA maximum) free chlorine is not enough to keep algae from growing. Trichlor can be used as a sole chlorine source but you need to keep higher and higher levels of free chlorine as the CYA level goes up.
One recommended strategy is to use a combination of liquid chlorine and trichlor until the CYA level is 50 ppm then switch to any source of chlorine without CYA (any hypochlorite). This will ensure that green water from algae is under control by proper free chlorine levels.