Do you think fish can cognitively differentiate between 4ppm and 6ppm?
I don't know the answer to that, but I have heard of fish "piping" at the surface when DO levels get low (how low IDK). At some point they know to head to the surface. A chart in the following Bob Lusk article says that the minimum acceptable DO level is 5 ppm. My pond seems to spend a fair amount of time at or below this point. So far, no piping...I just assumed that the fish would prefer better DO levels and hang out in that upper 2 feet. I never see them there, they are always below the clarity range.https://www.bassresource.com/fish_biology/oxygen.html