Sorry for the brutal experience! Thank you for sharing that with us though, it's always good to be reminded of the dangers, no matter how obscure! I'm guessing the boiling water mixed with the toxins to create a deadly steam.
I have a few paly's in my tank as well, brown and green, not particularly nice. I never really liked them, but I can't get myself to kill something just because it's not pretty...however, if it might be dangerous, it might be time to leave that rock out in the backyard (and possibly rid myself of racoons too ... j/k!)
Thanks for posting and reminding us all to be careful around those little chemical factories!
V