What kind of fish?
How long since when they were last seen health and when they perished?
What other fish do you have?
The necroris is rather extreme making me think if this happened overnight and based on the extent of the decomposition it might indicate there was a helper in the tank like some ghost shrimp and perhaps this fish died naturally and was rapidly broke down by something else other than the original cause of death. I honestly have never seen this kind of necrosis in a diseased fish including ones with severe cases of fin root and popeye.
Just my two cents