HELP!! What happened to my Blue spot???

[naesco]

Well you do know what you are doing.
If your tank did not overheat the only other thing I can think of is lack of oxygen. Do you have lots of current in the tank and if not try a small powerhead just below the water line.

Jawfish is not a fish that is difficult to catch in the wild so cyanide in not normally used.
But twice in a row points to choosing a different LFS eh