You might not have the parameters stable. You say that you add salts &buffer after the water change. But, what is everything reading before the water change & then before you add any buffer/salts.
Your parameters could be jumping all over & stressing the fish.
I test the day after each water change and I have it down pretty good as far as how much salt/buffer I need to add to keep it all stable.
But, this is a good point as to why things have been getting much better than in the past. It may have just been major stress that was freaking them out. Maybe....
When I first set up the 85 gallon tank, it was a lot warmer and I was using pre-water softner water that I let sit in the sun to come up to temp. I used the outside spicket for the water. This wasnt a big deal as it was only 3 buckets at a time. But, now that I have the 140 gallon going and its winter time... I need to use heated water for the water changes.
Maybe as I get even better at adding the salt and dont have to adjust anymore... things wil level out even more and no more stress?