Yea, I would get my chlorine shock from a store called Ocean State Job Lot. They sell the 12.5% stuff by the case of 4 gallons. Since my CYA at the time was 40, I had to add 4 gallons of the stuff to bring my free chlorine up to SLAM level. Then kept adding throughout the day and subsequent days after that to keep it there. It was rough, took about a week and a lot of chlorine but once it passed the tests, it’s been good ever since with minimal chemicals. I have recently this summer switched over to a SWCG and love it.