Flukes are parasites on the fish body that are translucent, invisible. If a fish or a recent deceased fish gets immersed in freshwater, they turn pale white and fall off. There is a good YouTube video about it. While it needs to be done correctly as it can be stressful to live fish, it can also be done as a postmortem. And yes, it’s just another way to loose fish. Bit more subtle and prolonged than your typical ick/velvet. Jay Hemdal in the disease forum is an absolute specialist about those.