Exactly the right place to post it!

CYA loss can have 3 end products: nitrogen gas (no problem!), nitrates (not necessarily a problem) or ammonia (the PROBLEM!). The easy way to see what you'll need is simply to shock it, and see what happens. Is your pool really 20K gal? That's small for a 22x40, unless you have no deep end.

If 20K is correct I'd add 5 gallons of 8% bleach tonight, and then test again in the AM. The pH should be raised, too. I'd start with 2 boxes of borax. Low pH makes the byproducts of chlorine oxidation nastier.

If you raise the pH, and add the bleach (about 15 - 20 ppm on a 20K pool) . . . and you have ammonia, you'll have little or no FC in the AM, but may have a high CC. If the pool is slimy but has no ammonia, you'll lose some of the chlorine, but should still have some, and not so much CC. If the pool is just a bit dirty, a lot of the chlorine should last overnight.