A pond that was built in the 1960's that has not had any aeration or artificial circulation will more so than not, have some sort of Hydrogen Sulfide build up.
Why risk it? If you have the ability to automate the system as you described, place the diffuser at the depth that encompasses the majority of the shape of the pond. Then run the system the first day for only 15 min. and then off the rest of the day. The second day will be 1 hour, the third will be 2 hours, the fouth 3 hours and add an hour each day until you are able to operate the diffuser for a continuous 6 hours. After that you should be able to run it 24 hours a day if you would like without any concerns about killing your fish.
George and Cecil...I like seeing the way people adapt their systems.
