With a zero chlorine, you need to get bleach in it today--NOW!

If the water is green, go ahead and shock the pool per Watermom's instructions. You'll need to test as often as you can during the day, and add whatever amount of bleach you need to get back up to the shock level. The more consistent you are about keeping it at that high level, the quicker the pool will clear.

The reference to sundown that Watermom made was for testing, after the pool has cleared, to make sure all the algae is gone. If you test one night and again the next morning without having added any more chlorine to the pool, and don't lose any in that time, then you're finished shocking and can let the chlorine drift back down. But while you still have green water, you just need to test as often as possible and focus on maintaining that shock level of 20 ppm.

Janet