The tannins are thicker than I'd like for viewing, but it seems they are a regular seasonal occurrence here, so I wonder if the temporary anti- bacterial properties help purge the fish of certain natural microbial parasites, or populations or bacteria that could become pathogenic if left unchecked.
I'm surprised that the pH of the tank hasn't dropped in any significant way.
Normal pH is 8-8.2, and when I tested for pH the other day, it was much closer than I expected.

I ran both low and high range pH.

The jug of tank water is just before I started daily diluting.