Don't be too quick to blame it on aeration alone. You have a beautiful pond and from the picture above I will make a rough stab that it is just about an acre? If it is typical depth for a pond that size and gradual sloping banks down to depth then your amount of water may have easily completely turned over with 12 hours of aeration using 2 diffusers going at once (assuming good quality membranes on those diffusers making millions of bubbles and that the membranes are clean). If you completely turned your water over in 12 hours then it is unlikely that an O2 crash was to blame.

Perhaps a fish kill immediately after taking a stagnant pond and turning on the bubblers for the first time and running them to long can be blamed on the bubblers, but it sounds as if you had the pond cleared up and had aeration running for quite a while before this kill event.

Superheating can be a problem with daytime aeration running but you would have to get temps from your surface and at depth during the day while running aerators during day to see if that is a reasonable theory or not.

A vertex dealer on this forum can probably help you with the math to figure out gallons in your pond and gallons turned over by your particular compressor and brand diffuser disks.

Your pond looks great, think positive, I would try to nail down the details of volume, need for aeration, is 2 diffusers enough for 12 hour runs vs 24 hour runs and if you can get by with 12 hours then you can stop blaming yourself, you can run at night and save electricity and still feel good about everything.