Ron, that is, I'm afraid, a sign of an internal bacterial infection. This arises when fish are fed contaminated food accidentally. It is usually the case when a fish consumes a worm or another live food that was carrying a parasite, in this case, the deadly bacteria. Do a simple autopsy on the affected region and you will know what exactly killed the little guy.
I had the same problem with my Corydoras cats when they first came in from the importers. Most of these wild corys do have an infection or two but it usually clears up during quarantine. I would suggest that you quarantine all the fish that had come into contact with the dead body. There is a remedy for this internal infection. Its called Sera Bakto-pur. There's a tablet form for the fish to feed on which helps to clear the digestive tract of unwanted pathogens.
Bookmarks