There are multiple "tannin reports" on Pond Boss that are binary. Either, most of the fish died when the oak brush piles got flooded, or all of the fish appeared to do just fine.

I have personally caught LMB from tiny ponds with black water where the oak forest was encroaching on the shoreline so closely you could barely cast. Yet other people have reported that when their pond picked up a light stain they then suffered a large fish kill.

I suspect some aspect of the pond water chemistry affects the tannin molecules and renders them inert in terms of harming the fish in some of the cases. However, I have never read on Pond Boss (or elsewhere) what water factors are important in determining tannin toxicity. That would surely be a useful bit of information to have!