VacavillFC3S, agreed heh, but I was thinking primarily from stuff I read before, like if you use certain household solvents, not even in the room but say the next room over, there was something in it (ammonia...? hmmm) that would just kill off the mantis, but everything else would seem fine. Roy just posted something about copper being very, very lethal to mantises, lower than detectable levels by test kits. That was the basis of my thoughts, but I agree that without knowing specifics, or even naming specifics, it really is a crapshoot hypothesis heh.
But yeah J2FcM, try a water change and hope for the best. Hopefully Roy can chime in with his insight and, also, that your mantis comes out of this a-okay.