Tank mates can make it behave differently. There can either be no need for it to get its aggressive coloring or it may color up completely because it is the alpha in an army of 1.
did you ever consider it being a red front? but it is probably the PH, and the PH would not kill your other fish if it is at 7.6 if anything you other fish would like it.
Changing your PH so much in a short time can kill any fish. Don't use PH up, add a small amount of crushed coral and that will bring it up over time.
The problem isn't just the fish, it's the bacteria. When you change the PH or even the temp so quickly you can lose your beneficial bacteria, and we all know what happens after that...