You have the same problem we have. I made a homemade aerator a few years ago that helped a little bit but the pond turned over a couple of years ago and killed the fish. The major problem we were told is that our pond sits down in a hole out of the wind just like yours is. Supposedly for proper aeration there needs to be wind movement over the surface of the water to create constant turbulence. This is what creates proper oxygen levels to prevent aquatic life from suffocating.
You have the same problem we have. I made a homemade aerator a few years ago that helped a little bit but the pond turned over a couple of years ago and killed the fish. The major problem we were told is that our pond sits down in a hole out of the wind just like yours is. Supposedly for proper aeration there needs to be wind movement over the surface of the water to create constant turbulence. This is what creates proper oxygen levels to prevent aquatic life from suffocating.
I am going to swap to a sprinkler style in the spring