Re: help adjusting
In a 21000 gallon pool 2 lbs of stabilizer will only raise the level by about 10 or 11 ppm, not enough to show up on the test. I would start with 5 lbs initially which should raise the CYA by about 30 ppm if your pool is 21000 gallons. Even if you are not at 0 ppm now but it is too low to measure you CYA won't be too high if you are not using trichlor tabs but use bleach.
Add the CYA to the skimmer with the pump running and don't bother to test it for a week and don't backwash the filter for a week either. In the mean time keep adding bleach to maintain at least a 3 ppm FC level in the water.
A more important question is what is your combined chlorine reading after adding chlorine. I am assuming that when you say chlorine is 0 you mean FC. Telling us both FC and CC readings and how long after adding chlorine (and how much added) will tell us a lot about what is going on in your pool.
Retired pool store and commercial pool maintenance guy.
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