Blaine this is a great question of which I probably dont have a great answer but here is my take. In small ponds aeration can change things quickly as they usually have more turn (lift)capability than larger lake systems.So in smaller ponds 3 acres or less that have systems that are sized to lift at least once per day just start them up prior to stratification and prior to oxygen demand out stripping supply. This will vary from pond to pond even within the same area and can vary from the same pond year to year.In central N Indiana you are probably having the same temperature swings we are and have more natural circulation than most aeration system could provide.I started mine up a week ago when it was 70F and currently have them shut down. On the larger ponds we service I have about half of them running and the remainder will get turned on next week.Normally you are safe in northern Ohio and Indiana to start them mid April. If I see prolonged night temps predicted in the low 30's and less I turn them off until I have 40F plus nights.DO levels should be very high with all this wind and cold water and demand should be low. Im guessing most DO levels here 10-12 ppm but I have not checked.and yours are probably high also. Pehaps Cecil has checked his readings.If you have only been aerating for a season or two and have a pond that is 8-10 years and older I would do a slow startup for the next few seasons, Keeping in mind that some systems are lifting a lot of water and some very little plays a huge role in answering this question too.