Sorry to hear about your fish kill.
I am NOT an expert on this topic, so I am just repeating what I have learned on Pond Boss.
If your fish kill is due to a low oxygen situation, then it is typically the largest fish that suffer the highest mortality rates. (I think it is a surface area of gills to body mass ratio issue, which gets worse as a fish gets larger.)
Lots of Water Shield usually means lots of shallow water. It also means very poor wind mixing due to the surface layer of your plants. At the start of August, you probably had very warm water in your shallow areas plus unoxygenated, unmixed water in your deepest spots. That is perfect conditions for a cool summer rain to cause your pond to turn over and put your fish into that unoxygenated water. Further, your pond water probably gets most of its oxygen from the Water Shield, rather than normal wind mixing. Several cloudy days in a row, either before or after the rain, would have shut off your oxygen generating plants. Finally, when your algal bloom died off, the decomposition process actually CONSUMES dissolved oxygen in your pond.
I think you got hit by all of the possible "bad" things at the exact same time.
Can you go out to your pond right at dawn? That will be the lowest oxygen point of the 24-hour cycle. Can you observe your bass and BG piping at the surface for oxygenated water? If so, you are still in the middle of your low oxygen event.
I do not believe that your Water Shield killed the algal bloom, I believe it was more likely just part of the bloom boom-and-bust cycle. However, I am pretty certain you do NOT want to kill your Water Shield right now. That will only make things worse with additional decomposition of plants in your pond!
Lots of speculation above on my part. Hopefully we can get some more expert advice into your thread to give you some better guidance.
