You can add bleach in the evening to bring back up to 15-18ppm if you want and then test in the morning before the sun rises. If you have held the chlorine overnight, then there is nothing in your pool that is using the chlorine. If you have a drop overnight, then there is something in the pool that needs to be sanitized - at which point you would keep your chlorine at 15ppm-18ppm until the level held overnight and then let it drop to maintenance levels.