What could be so toxic so quickly? When I've had massive fish die-offs it was usually coincidented with moving my ritteri, so I assumed it was shed nematocysts that caused the fish to suffocate.
Anyhow the point being that it seems to me that suffocation seems to be a likely cause for rapid onset, multiple deaths.
In this case, we're talking a few hours after adding a few rocks to a large water volume so I would guess it not to be an ammonia cycle. You mentioned there was a smell to the rocks - I wonder if it could be H2S. It would fit the profile of suffocation.
Anyhow I'm sorry if you're not ready to speculate on causes - I can only imagine what you're feeling right now - I'm so sorry.
