My morning DO levels are typically between 4 and 5 ppm from the surface down to as far as the meter cord will allow (3 feet). I would bet that those readings are consistent throughout the water column due to aeration.
Evening DO levels tend to be below 4 ppm below 24" deep. Above that 24" mark, they are regularly in the 5 to 6 range. I never see any of the fish in this part of the water column enjoying the extra DO???