How big was the colony of Zoos? I remember a while back that Murminator was giving away big chunks of it and one of the people that grabbed a piece had the misfortune of livestock dying off on him as well.
The speculation was that perhaps the piece of Zoanthid was too big (in your case, a 12g) and caused chemical warfare within the tank. He tested his water as well and everything read okay, but I'm assuming that since it's coral secretions, that's why your water tests didn't read any of it.
Hopefully that helps.
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