Charlie are the numbers PPM or %?
If PPM, your meter is not calibrated correctly because water at 84ish degrees can't hold that much O2, it is just not possible (laws of nature). If that is % then you still have a O2 problem.
About your questions. A surface aerator should be able to add enough O2 to prevent a fish kill. The 1/2 HP Kasco you have should be enough, but with the vegetation dieoff, no one really know, because we don't know how much dead material is in your pond.
Continue to run the surface aerator 24/7 it is the best way to add O2 to the water.
