Yes, on a really hot sunny day, you can easily lose that much chlorine. CYA of 150+? Wow. So, you decided not to do a partial drain but just live with it? You're gonna have a hard time keeping algae at bay, I'm afraid. Just look at today's levels. You're down to 4.5. Dropping that low is an invitation to algae. I hope I'm wrong, but I'm afraid you're gonna have a tough summer keeping cl levels up high enough to avoid an algae bloom. I think you better plan on taking the cl level up to 15 tonight instead of just 10 so maybe by evening tomorrow, you won't have dropped below 8.