How did you get a CYA reading of 120? With it that high, you are getting algae because you are not keeping your chlorine high enough. Take a look at the Best Guess Chlorine Chart in my signature below for more about that.

With high CYA and the high chlorine levels you will have to run, you need a kit that can measure chlorine higher than 5ppm. Order a Taylor K2006 or 2006C. Link in signature. (Won't be available locally.)

What are you using for chlorine? No more trichlor tabs or dichlor powder. They have CYA in them and yours is already too high. Better to just use plain unscented bleach.

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