Hello... I'm newbie here so I'm not going to be much help but wanted to say that I sympathize with you. My situation is similar to yours (albeit, with a smaller pool... 28k gals is huge!), I'm fighting the algae enemy!
I'm also failing to see improvements after shocks.. but my understanding is that we have to keep at it, since it can take easily a week in some instances before getting significant improvements (?). Seems to me that you are doing all the right things.
Do you know the amount of stabilizer (CYA) in your pool? My understanding is that, although it definitely helps maintain a high level of Cl, this can also greatly limit the sanitizing effect of Chlorine if it is too high... Might be worth a look!
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