Let me clear something up about the vinegar test,
Certain rocks can contain CaCO3 which can cause the Ph in the water to rise, CaCO3 is an alkali compound which acts as a neutralizer to the acid (vinegar), when an acid is poured on the rock the calcium carbonate neutralizes the acid therefore making it bubble. This means that just because a rock bubbles when vinegar is poured on it doesn't mean that it is unsafe, it just means that there are traces of calcium in the stone. Most rocks that contain traces of Calcium contain just a small amount and isn't often a large amount to even notice, so I wouldn't worry about it changing water levels drastically.
Please do not boil rocks, certain rocks can have small air pockets in the stone which when heated can cause it to expand and explode. Just pour boiling water over the rocks and soak them instead so there isn't a constant heat on the stones but a steady cool down instead.