... Am I chasing the meaning of life?
No just the answers to your questions.
Here is a question of my own to anyone who can explain. What is the chemical process by which "low alkalinity water" becomes more basic than "high alkalinity water" in the graph above.
QA, One thing stands out to me in your graph. Your BOW's PH is never alkaline ... its always acidic ... all day all night? I also noticed from a different post you made that your phosphorous is always zero. How is your primary production? Do you get good algal blooms ... just saying ... that if there isn't enough algae ...
If the condition were a fish standing weight greater than the algal biomass can optimally supply with oxygen then you might get at least some of the conditions you are seeing. The fish metabolism would respond to lower levels of oxygen by slowing down. I think I would get the water tested and get some recommendations for water fertility.
A PH modifier (like lime) might be front and center on any recommendations made.