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.
