Thank you all.
What a stupid mistake!!!! I did ask on the forum about the water a couple times, but no one replied so I assumed it was OK. And with the pet store saying it was fine, I just assumed....
I'll be getting the water conditioner right away, and until then will use bottled spring water. The bottle says 'spring water', so I'll assume it's safe until I can get the water conditioner.
So no one seems to think it's necessary to completely break down the terrarium and clean it out?
Everyone believes it was the water or a poisonous insect and not a contagious illness?