Disregard what I said above.

I thought you were circulating the pond with a surface device.

I checked your link, and saw the Kasco's operate by spraying the water into the air.

I guess you could get a little evaporative cooling of the water while it is in the air. I wouldn't think that would be sufficient to set up a convection cell to move your deep cool water out of the bottom of the pond.

However, it doesn't matter what the "theory" says, if your experiment is showing different results (the deep water is warming), then you have to follow your experimental data.

Perhaps you should move the aerator to the shallowest spot in the pond where it will operate, without stirring up sediment from the bottom.