This happened to many of my freshwater fish as they died from gill flukes, the flukes damage the gills which reduces the oxygen in the fish's system and they kind of go delirious like a person in the high mountains; they just gradually lose consciousness and their cells break down at the same time. I'd be surprised if your fish had flukes and they were not "scratching" themselves on rocks and other things to try to remove the flukes because they are otherwise helpless. Are they flashing or scratching themselves on anything?