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They may call it stabilizer on the 6in1 strips. CYA slows down the action of the chlorine. A little is good too much is bad. It is added to the chlorine you are using dichlor as it is used in pools and sunlight removes it.In a tub it doesn't see much sunlight so the CYA builds. I use dichlor until it reaches 50ppm and then switch to household bleach. When the CYA drops to 30ppm i switch back.