addy:
It would seem that if the water parameters are normal, if food was not fed too late, if no toxins are evident in the water, that one would have to consider 'loss of oxygen' or some pathogen.
I don't think fluctuations of temp would affect much at all because the water takes a long time to change, especially if there's any pond depth at all. Two weeks of 50s isn't going to melt 8" of ice that fast(I know, you didn't say he had that much ice; I was using an example. If he had less ice, such that it would melt and he could see his fish moving, ice cover was not that thick and hence, temps not that cold, either. Certainly not close enough to freezing if his fish were moving around and seen that easily, figuring that they should be at the bottom, 6 feet down), let alone affect the bottom water's temp in radical/sudden shifts. Especially of there's no pump moving water.
I do wonder where some of the water/pond experts re chemical analysis and systems process are, re weighing in on this thread. I'm certainly not one of them but I've read enough threads to know there's usually a few here that like to pontificate exponentially. Seems some of that knowledge might be of use at the moment.
From my researching, it would seem that this is a 'winter fish kill' and its conditions must have somehow been reached. The 'somehow' would be the question. Maybe there's a fine line between how much decaying plant matter was in his pond (as in; how much is too much and how much is adequate for fish to be sustained during the winter--is there some measurable amount that can be easily determined and therefore prevented for the future, or is it more subjective without such testing?). The one article I read indicated that large fish would go first in a winter fish kill; of the survivors, were they small or large?
If the aerator was in the upper portion of the pond (as I'm led to believe is proper, re the research), the lower portion might not have continued to maintain its share of oxygen and that's where the fish would have been. If there is stratification, maybe the oxygen did not disperse evenly. That said, I think I read that stratification begins near a depth of 6' (I'd have to check that number) so not sure if the bottom layer would be both colder and richer in oxygen than the middle or top.
I guess it's now easier to go with the test fish case, but I'd still have wanted an autopsy, just to be sure. He may not know the results of this test fish case until NEXT spring (and stocking his pond to the same limit as the recent past), and that's if the weather conditions are similar or worse. I think one can look at a situation and though everything looks fine, be right on the line of disaster without 'seeing' it visually.
Just some further thoughts.
>>>addendum; addy, I reread your replies and you say he saw his fish in mid Jan/early Feb, that they were healthy and looked fine. You also say this was after a thaw, then he was hit with more cold. You also note his water is greenish, indicating either algae (as I think you guessed in your post) or perhaps more decaying plant matter than assumed? In any event, algae or more decaying plant matter, wouldn't both have taken the oxygen out of the water, depriving the fish? Especially if you think there was an algae bloom; all that algae suddenly exposed to freeze and dying; there'd be a lot more decaying than the fish could handle, perhaps. I would think the water would be VERY clear if no to little activity had been occurring during the winter months. I know that even with what decaying plant matter I have in my pond, even in the summer, the water is clear, NOT greenish or cloudy. That fact all by itself tells you something was wrong re water conditions. And yeah, you'd think the ammonia would be up if he tested the 'unclear water', but this is where I'd have hoped some of the water/chem experts here would get involved. Like I said, I'm not an expert.
Just some further thoughts v2.
Michael
