Most saltwater aquarium layouts prominently feature rockwork. Rocks give fish places to hide and help them feel safe. Additionally, rockwork provides a place to anchor invertebrates like corals. However, your selection of rockwork matters, since some types of rock can affect water chemistry, either purifying the water or contaminating it.
Live rock consists of old, dead coral skeletons that have been colonized by other organisms. Most importantly, live rock contains bacteria that break down fish waste, helping to provide biological filtration for your aquarium. High-quality live rock often contains other organisms, including desirable encrusting algae, sponges and ornate worms. However, you need to cure live rock, or purchase it already cured. Curing is the process of letting delicate organisms killed in shipping rot away before adding the rock to your aquarium, where they can foul the water.
Special saltwater aquariums, called reef tanks, feature coral and other invertebrates. These organisms absorb calcium and other minerals from the water and use them to build their hard support structures. In such an aquarium, you need to replace the calcium these organisms absorb. You can do this by using calcium-bearing rock, like limestone, sandstone and tufa rock in your aquascaping. These rocks slowly leach calcium and other minerals into the aquarium, mimicking the process by which these minerals enter seawater.
You also have the option of using chemically inert rocks. This is an unusual choice for saltwater aquariums, but it will not hurt anything, and may create a unique-looking aquarium. A wide variety of rocks have no effect on water chemistry. This includes rocks like quartz, slate and granite. Many pet shops sell these rocks for use in fish tanks.
What to Avoid
You should always get your rocks from pet shops. Never collect your own rocks. Some contain toxic metals and elements like arsenic and copper. Many marine organisms, including sharks and invertebrates, are particularly sensitive to dissolved metals. Additionally, a rock may have been sprayed with herbicide or pesticide, which would wreak havoc on saltwater critters. Even rocks from hardware stores or landscaping depots may have been preventatively sprayed with various chemicals. Pet shops sell rocks that you know are safe.