First off, you need to make sure they are sable and wont mess around with the pH. The first test is to drop some vinegar onto the rock, if it fizzes its out. If it dosent theres one more test. You need to get a bucket of tap water and lit it sit for 24 hours. Then test the pH of it. Then add the rocks. In another 24 hours you should test the water again. If there is a change its not a good idea to use them in the tank because they may mess up the pH. If they pass both tests wash them with a hose and a cloth or something. Pour boliing water or bolling them will also help. Then add them to the tank and enjoy.