Last night after you added the bleach did you wait 1 hour and retest? Or did you only test the chlorine this morning? You should test it after adding (wait an hour) cause you might need to add additional bleach to reach the right level - see above post. Then in the morning, if the chlorine dropped more than 1ppm in the darkness, you know it's NOT from the zero CYA, it's from algae or something in your pool, and you need to keep adding bleach to 10ppm to kill it, does that make sense?