Frustrated,

Guess what! All the times before when you added "shock", you were actually increasing the chlorine to many times higher than 3. That's the whole point of "shocking".....getting the chlorine high to kill (or shock to death!!) the evil algae or microbes in the water. The 20ppm is just for the shocking level. Here's the chart below as a reference.

Stabilizer(CYA) . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . .8 ppm . . . . . 15 ppm . . .. 25 ppm

Most people don't swim during the times that they are shocking the pool, because they are trying fight off contaminates in the water. Once the water clears and contaminates are killed, then the chlorine level is allowed to go back down to maintenance levels. In your case with a CYA of 90, you'll need to stay between 5-10.

Also chlorine is chlorine, it just comes in different forms and many different kinds of products. I used to use pucks, which provided the chlorine for my pool. Now I use bleach, which provides chlorine for my pool. Some people use granular pellets in a floating device, which provides chlorine for the pool. IT IS ALL THE SAME CHEMICAL COMPOUND IN THE END, and your pool doesn't care which one you use, as long as it gets its' sanitizer.

How's the pool looking? Is it clearing? What are your current numbers? Good luck.

CaryB