It depends on the make-up of the rocks,Calcium containing rocks will have a considerable affect on the pH of the tank. Some crystalline rocks will dissolve in an aquarium. If you do try to use garden rock first soak it in a bleach solution,if it fizzes or bubbles it won’t work in the tank. If it passes this test soak it in the bleach solution for a couple of weeks,then rinse it thoroughly and soak it in a Sodium tri-phosphate solution for a few days. Then repeat rinses ’til you can’t detect any chlorine odor.