The chlorine tablets that you're using are what's causing the CYA to spike, and what's causing your pH to be low. I would stop using any stabilized form of chlorine and use plain, unscented bleach. You do need to shock the pool, since your CYA is 100, that means getting your chlorine up to 25 ppm and holding it there until the pool clears by testing and adding more chlorine (bleach) as many times daily as possible. In a 30K gallon pool, each 2.5 gallons of 6% bleach you add will raise your FC by 5 ppm. Each 1/2 gallon will raise it by 1 ppm. Keep the filter running 24/7 during the process, and clean the filter as indicated by your pressure rise. Once you can hold chlorine at the 25 ppm mark overnight, and the pool is clear, then you can let the chlorine drift down, but never below 8 ppm since the stabilizer is so high.

Janet