I remember having one rock do a MASSIVE die off on me in the inital stages, and after that, the tank, water, everything smelled like rotting corpse.
It was ludricously bad die off that I had to remove the rock and get another one from the ocean (I live in Hawaii). Took it to a friend who knows and he said the rock was actually a piece of very Old coral that appeared to be rock.
Moral of the story, not saying you have a piece of sponge or rock but die off can be BAD.
So there you go.