I am not an expert in treating or diagnosing pond fish kills BUT I am trained as a biologist. By understanding the common causes of summer fish kills it may help you diagnose the problem.

The most common cause of oxygen depletion is a die-off of macro and microscopic plants. This is most likely to occur during hot overcast weather. During this type of weather photosynthesis can be inefficient and plants often consume more O2 then they produce. Eventually they begin to die and decompose further depleting O2 levels. Could the small stream have been providing nutrients to your plankton? Did you notice a dieback of plants?

Another cause is a water overturn. Water can become stratified in the summer. As the temperature difference between the bottom of the pond and the air increases, wind is unable to mix the layers. The upper levels contain O2 while the lower levels will have very little. A tremendous rainstorm or something can mix the levels and cause depletion. Could the decrease in in temperature Wednesday morning have allowed some mixing of the layers? Was there also a strong wind? I would guess that if a pond was the susceptable to stratification it would be severe this summer and a few hours of cold weather could not be a problem but the theory is fascinating.

Don't forget to rule out other causes of fish kill like some pesticide/chemical runoff or disease/parasites.

A question for others who reply; is it unusual for catfish to die first? Generally they are considered tolerant of low O2 levels. On the flip side, the largest fish generally will die first and they may have been the largest.

Good Luck