I have repeatedly fished a small pond (unaerated) in an oak forest. The water is always that clear brown/black color that I associate with high levels of humic acid. (I suspect it is mostly due to leaves decomposing rather than branches in the water.) The pond gets very little wind aeration because the oak trees crowd so closely to the edge that it is difficult to cast.

We have caught fair numbers of LMB and GSF (on bass lures). We never deliberately tried to sample the panfish population, so I have no idea what the forage population looks like. I never got to fish the pond enough to determine its "productivity". (I assume it has fairly low productivity due to significant tree shading, plus other factors.)

I took one buddy fishing at that pond, and he declared we would catch nothing once he saw the water. He then caught a 4# LMB on his second cast. (Which is pretty good for an unmanaged small pond in Kansas.)

Fish are darn tough critters - sometimes. They are also seemingly very fragile at other times.

I have absolutely NO IDEA what level must be exceeded of humic acid and tannins in the water to cause problems for the fish population!