In my last visits to the pond I have seen the cnbg dying off and the ones I see swimming are all under stress or at least that is what it looks like to me. Guess from the ice storm. No other reason why I am seeing them die off. They are mostly black in coloring and have some white spots on them along with maybe some fungus on them. I have not tested the water lately so maybe I need to test it but I see no reason why the water would change from it's baseline other than the ice storm. I did catch a lmb in the two-pound range and it looked perfectly healthy with good weight. I saw some other lmb that seemed to look good but the water is a little cloudy due to the snow and ice along with the recent rains. It has been tough on the fish the past 6 or 7 months.
I went through the white spot fungus on my larger CNBG after a cold snap in Oct 2017. So far as I could tell, they all died, though no other fish did. Chalked it up to spawning in shallow water that was hit with sudden chill.
Have you thought about adding some salt, or maybe antibiotic in the feed?
