When adding rocks to an established tank, you'll need to be careful. Make sure that you have already cleaned the rocks - usually a rinse with hot water is fine. I would wait until you need to do a water change to add the rocks and I would do it very slowly. If you're talking about gravel, I would add it by the cupful and take the cup of gravel directly to the bottom of the tank to deposit it. If you are adding larger rocks for decoration, I would place them slowly by hand. Adding new rocks and gravel at a water change makes it easier to move things around the way you want them. Remember, fish can stress easily and you want to make this process as easy as you can for them.