Jeff, what's confusing you this time?

Running an aerator at night is done mainly because it is a known factor that plants are not producing oxygen. There can be many more times when an aerator SHOULD be run in a pond.

Those plants are giving off carbon dioxide in low/no light conditions....(or dying and consuming oxygen if cloudy for a couple days). Running an aerator at night not only allows the tranfer of oxygen into the water as it circulates, the "boil" and circulation allow carbon dioxide and other dissolved toxic gases such as hydrogen sulfide (rotton egg smell) to be released from the water. This prevents/reduces acid formation and the resulting wide PH swings...all this equals better water quality and less stress on the fish.

Another good reason for running an aerator only at night is to minimize the temperature rises in a pond.....eg. circulating sun-heated water during the day.

Aerators most of us use are easily setup on a timer. Large fish farms will use extensive and EXPENSIVE monitors to automate and give full control over when aerators operate. In a recreational system, adding a lower end 100K+ automated system is far from practical, but say in a 10 acre production pond with CC stocked at 11,000 pounds per acre and worth $1.50 per pound....saving one die off PAYS for the system!