A couple variables stand out. 1. Fish gulping at the surface and all sizes dead next day. 2. Low pH of 4.7. This low pH value likely stresses the fish and makes them more vulnerable to other factors such as low DO. pH needs to be between 6.5 and 9 for healthy fish and invertebrate communities.
Cattle access to the water and maybe the stream can contribute lots of manure, and manure has nutrients (N & P plus others) to grow excessive plants that when decomposing, consume lots of DO especially at night. Is the water green with algae? Too much phytoplankton at night consumes lots of dissolved oxygen with lowest values at dawn. Decaying cattle manure also consumes lots of DO 24/7. Warm water holds less DO than cold water, thus the DO will be consumed faster ("less in the bank") when less is present to enter the night period.
You won't positively know the reason for the fish kill unless DO measurements were collected while fish were dying.
Stream water could be contributing to the low pH. Pollution spill upstream???? If pH of pond always 4.7 to 5??? This is not normal pH values for reproducing healthy fish communities.
