Quote Originally Posted by kelemvor View Post
One thing to note, is that at CYA of 100 or more 4ppm fc is not effective at killing algae and you will get a green pool eventually. You need to keep the pool at at least 8ppm (somewhere between 8 and 15) and you'd have to take it up to 25ppm to "shock".
The Best Guess chart page explains this.

Also, with a PF:7, your pool's FC (free chlorine) will be increased by 7 for each pound of chlorine gas equivalent added -- 1 gallon of 6% (chk %!) is about 0.5 lbs equiv, so will add about 3.5 ppm per gallon or 2.6 ppm per 96oz jug.

With a CYA of ~120, a dose of 3 gallons (4 96oz) would be a good start, possibly followed by 1 96oz jug per day, till you hit 20 ppm. Then, back off and hold in the 12 - 15 ppm range.

With CYA > 100 ppm, 20 ppm is NOT too high to swim. I've been running a large (>200,000 gallons) country club pool all summer at CYA in the 70 - 110 range (leaks a lot) and FC in the 8 - 20 ppm range . . . with NO complaints whatever.

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