OK,

That's very revealing. Thank you.

You are clearly fighting something in the water. That's why it's dropping so much. Of course, as I said, EVERYONE around the nation is using large amounts of chlorine this season. You need to add sufficient bleach to bring your FC up to shock level: 15ppm, and keep it there for at least 24 hours or until it stops dropping drastically. It may be as little as 24 hours. After raising the FC, check the water 3x a day and raise it back to 15 as needed. When, after 24 hours, it doesn't drop, or doesn't drop much (especially overnight, when there's no UV to break chlorine down, you can gradually let the FC drop back to the maintenance level which is 3 to 6 ppm and which is where you'll keep it on your normal daily basis.