I have no experience with fish that have died due to low DO but I have seen fish die and float at my place. When my Tp die due to cold water as the water temps drop, they have a fungus that grows on them or what I think is a fungus. And your pic of the Bg has the same look with the patch that looks like fungus to me. I have also had the same or similar type of fungus on some of the Golden shiners in my sediment pond when we get large rain events. I think in both cases it was brought on due to stress. So, I would look for something that is different from your norm. Pat, may be right when looking at the grass matts. Maybe a weed killer is in the matt and was washed into the pond during a rain event. In my sediment pond the water will turn color with all the clays being washed in and that causes stress, so any large rain event recently? I may not be much help here but I hope things improve for you soon. Dead and floating fish in large numbers can't be good.
