The first thing that comes to mind is the chemistry of the new water. Did you test to make sure the salinity and pH of the old and new water matched (or at least were close)? I've seen (as well as heard anecdotal reports) that the mixing instructions on salt mixes tend to be inconsistent. It is also possible that the salinity and pH of your tank water had shifted due to evaporation, top offs, etc. to not match the specs of newly mixed water, and that could be causing stress. Also, when you mixed the new water, did you let it sit (or, better yet, circulate) after mixing and adding dechlorinator, or did you use it immediately? If you didn't let it "age" it's possible that the dechlorinator didn't have a chance to fully neutralize the chlorine before being added, and I've heard that freshly mixed saltwater tends to be somewhat irritating if used immediately after mixing, but I'm sure opinions on that vary.