Fish are kind of like over sensitive people if anything is out of whack then they start to fret about it and they get stressed from anything being out of whack that is why a bigger tank is better. A bigger tank tanks little thing that go out of whack and can adjust on it's own some times and other times all you need to do is a 10% water change and everything is back to normal. I have an angle that killed a brain and my tank just adjusted to the change in the brain dieing I try to catch the angle to no avail so I will wait and try a bottle trap latter it does not pick on anything else. It did try and pick at a monti but the polups retract and the fish got nothing now leaves them a loan.
Forgot to mention ICK starts in the fishes gills so they could be reacting to that a fresh water dip before QT will help alleviate them a bit get anything that is on the gills but loose off.

There may have been something that powdered ALK dose stuff that has set off your tank a chemical unknown or not specified.

Bill