Can a rock change something in the water and kill the fish?

[Reventon]

My friend put think big nice looking rock in his aquarium and 2 weeks later most of his fish died, is it possible that this is because of the rock?
If it is then how can I make the rock safe because I want to put it in my aquarium.

 

[mqktandy]

Limestone can contain all sorts of stuff that can do harm to your aquarium from changing the ph to leaching oil into the water

 

[GhostShrimpMan]

Limestone can contain all sorts of stuff that can do harm to your aquarium from changing the ph to leaching oil into the water

Yup, certainly possible. Post a picture of the rock to ID it. If it is limestone I think it fizzes when exposed to vinegar.

 

[Reventon]

Ok ill go to my friends house tomorow and get a picture

 

[brich999]

also can mess with hardness of water depending on type.