With almost no stabilizer present, 20 ppm of chlorine is rather high. FC=20 *and* CYA=0 is high enough to damage swimwear and be irritating to some swimmers.

BUT, if you *really* have no stabilizer, you'll lose almost all your chlorine in an hour under full sun. So, that's a sort of the test. FC=20 and CYA=20 is very different and NOT a problem, even if still rather high.