Ok, so the latest details on my struggle to save my pond/fish...
After some pretty heroic efforts to aerate the water using untold gallons of gas to power a 200GPM trash pump, 100' of 2" fire hose, and a fire nozzle, along with various other pumps and spray nozzles, the fish that were swimming at the surface started to disappear into the murky water depths.
In all, we've lost 10 fish, 3 of the >20", 3 of the 10" and 4 of the smaller ones. What is particularly creepy is that two of the small ones were disfigured. One was all bent like it had scoliosis. The other was missing an eye... not like it was removed, but it was born without one. Mutant level kind of stuff on both.
Maybe the gene pool is just a bit too shallow?
Anyway, today the test strips came in and this is the result:
It appears to these rookie eyes that everything is in the ideal range. So I presume the gulping distress the fish displayed was likely due to a genuine lack of O2 in the water.
One thing I did notice is that the "slimy" white/clear foam on the surface turned green as soon as the sun hit it. In just a couple of hours, the surface was almost entirely covered by these tiny little green circles. Perhaps this is what is consuming the O2? Blasting it with water from the firehose made the green vanish. So I kept the pump running during daylight hours.
Hopefully the Lilly pad transplants and the pending filter installs will reduce the chances of this happening again.
All the pointers were much appreciated!