I took three fish to Auburn University to be examined and 3 water samples. The water samples were from the pond, from the stream entering the pond, and the stream below the dam that i used to top off the pond during dry times. Below is the final answer that I received.
we confirmed the bacterial isolate as Aeromonas hydrophila, a common opportunistic pathogen. We didn’t see anything unusual in the water samples although the pH was a little low (6.3) but that could be due to organic decomposition occurring between the time you took the sample and the time we measured it.
From our analysis, I can confirm that at least one of the fish had a systemic bacterial infection but because of the nature of the pathogen, I won’t recommend antibiotic treatment.
I’m sticking to my gut feeling that you had too much organic matter in the pond due to either high stocking rates or too much feed (although you probably weren’t feeding them in January, right?). It is important that you remove as many fish as you can (and bury them, don’t leave them out in the open) and try to reduce the organic content in the pond (I know that’s easier to say than to do).
Be careful when you handle the dead fish (wear protective gear). Aeromonas hydrophila can cause infections in humans, it’s rare but it can happen if for example you puncture yourself with a fin/spine.
