The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
Well there is a test you can do. Pour vinegar on the rock and if there is NO fizzing you are safe. Most rocks will effect the pH, so make sure you put the rocks in while it is cycling so you can get the pH stable before you add fish. I stay away from the rocks that look rusty, or that are very porous. I use a lot of slate, it's plentiful in the in my backyard.