There are different views on this, but in general, as long as rocks are sterilized and are not calcium based, you can put them in your aquarium. Scrub them thoroughly and either boil or soak them in a mild bleach solution – rinsing thoroughly afterward before placing them in your aquarium. Calcium based rocks are usually white in color and may raise pH and alkalinity to unsafe levels. They should only be used in African cichlid aquariums. To find out if a rock is calcium based, place a few drops of white vinegar on it; if it fizzes, the rock is probably calcium based.