The rock itself is almost always safe. What sometimes gets us into trouble is what is on the rocks that we might not see. Dirt and such can be seen and washed off but there are other things not seen so easily. Snail eggs, oil, pesticides, etc. can be really hard to see and still kill fish. Then there is always some chance that there is a fungus, disease or something else that could bother the fish. The easy way to make sure these are gone is a little scary to people who have not used it but many people do it all the time. If you mix bleach and water and soak anything going in the tank overnight, all the above questions are eliminated. After soaking, rinse the items and lay them out to dry totally. The bleach that is not rinsed off, will dissipate into the air. The chlorine in bleach is the same as the chlorine used in your water most places, so there is no need of panic about using it. We deal with it all the time anyway.