I am in Honduras. A fish in the AP system was found dead this morning. It had a hole in its side and looked to have been eaten from the inside out. It was fine yesterday. Any ideas about what this is and what to do about it will be very appreciated.
I recommend setting up a quarantine tank to place any more of your fish that appear to have skin ulcers, holes, and deep skin bruising appearance. Reducing and not adding to any stress on the fish is the goal. Keep the QT a bit warmer, cleaner, darker and, more oxygenated.
Try to identify any or all parasitic leeches, worms, or fungi present in the pond and/or local stream area. Salt bathing might help if caught soon enough and you could dose the whole pond if there are many other fish showing symptoms.
Try to examine the dead fish very closely. An autopsy should show what damage is done below the skin and to the fishes organs.
Take scrapings of the skin right at the lesion, about an inch away, and from a normal appearing area on the opposite side. Take deep skin tissue and organ tissue samples. Doing some microscopy research might bring you the benefit of knowing for certain what parasite or fungi did the damage. So you can treat for that particular pest in the future.
I hope this helps, and I'm sorry there can't be much more long-distance with so little to go on. Please keep us updated.
To me that doesn't look like a fish that was fine yesterday. I'd say it's been dead for a couple of days, but then again you're at a latitude where things rot very fast. The hole is from where the left pectoral fin was pulled out--possibly the cause of death or something that happened after the fish was dead and possibly depredation by other fish in its tank.
I would keep a close eye on the other fish, looking for signs of disease--much more informative than this rather far-gone corpse.