I saw something like this earlier this summer. I had some, not a lot, of 1/2" long clam-like muscles that floated around the pond. My assumption was that the muscles were floating to the surface with a minor outbreak of FA mats. I saw the muscles on the mats and I also saw them floating solo. It never dawned on me that they might have been dead so I never checked. This went on for maybe a few weeks. I raked some FA a time or two then the FA subsided and the muscles were not to be seen floating around anymore.
I never had any dead fish, but the pond clarity at the time, IIRC, was poor and there were bagillions of bullfrog tadpoles. It would not surprise me if my DO2 was on the dangerous side.
