If you have some organic load in the pool that is using that much chlorine, then shocking it only at night isn't going to win the battle--you need to shock it, and test 3-4 times daily and add more to get back up to shock level. It's maintaining that shock level that will kill off anything living in your pool. Letting the chlorine yo-yo up and down isn't going to accomplish anything if you're trying to kill some sort of bloom. That being said, I stick with my above post--I'm not so sure that your testing is giving you accurate results. What is your CYA level, and what are you using to test with?

Janet