What about the rock killed them? I did once read of someone blaming a rock they collected from a river for killing their fish, after a while it turned out they'd soaked it in bleach to kill anything then, after a quick rinse, thrown it in the tank.
Most rocks, especially those found in a river, don't dissolve very fast at all. They'll have been in the water for years, even decades, anything easily soluble would be loooooong gone.