The bloom used up all the O2 at night because the sun wasn't out, so the fish suffocated. An aerator would have eliminated that issue.
Cody note: An aerator may not eliminate a fish kill depending on the type and size. Most all aerators will reduce the size or extent of a fish kill.
Let me rephrase that. A properly sized aerator, run through the night most likely would have eliminated that issue.
Dr. Perca, how's that now? LOL
Water quality and knowing your biochemical O2 load on the pond all comes into play. For instance, take two ponds, with the same water volume and amount of fish in it. One pond has a dense algal bloom (higher biochemical O2 demand), another has 60" visibility. What one do you think would have the biggest issue in regards to low O2 levels right before sunrise?