What Kind of Rocks Can You Put in a Fish Tank?

Aquarists place rocks in aquariums for aesthetics and as hiding places for the fish and marine invertebrate animals. Aquarists should test rocks first by placing them in a bucket of aquarium water and checking the pH after a week. Rocks that do not leach minerals to affect the water chemistry are ideal for aquarium use. Rocks that are typically chosen for marine aquarium use are not suitable for fresh water tanks, because they cause a raise in the water pH levels.

  1. Live Rock

    • Live rock -- used exclusively in saltwater aquariums -- is not "alive," but is covered with a multitude of living organisms that grow on or within the rock structure. This rock comprises pieces of rubble that tropical storms and hurricanes have broken from reef structures. Beneficial bacteria colonies, marine sponges, coralline and other types of algae, worms, crabs and numerous other organisms grow and live on live rock. Such rock is therefore not only aesthetically pleasing in the marine aquarium, but filters the water biologically.

    Sandstone

    • Sandstone consists of tiny grains of sand and other minerals. This is a sedimentary rock that is composed of feldspar and quartz. Sandstone is available in various colors, including tan, red, yellow and white. Sandstone is resistant to weathering and frequently used as décor in fresh water aquariums.

    Lava Rock

    • Lava rock is durable and available in numerous colors. This rock is popular for fresh water aquarium use. Lava rock, also termed igneous rock, forms when molten rock from volcanic activities cools and solidifies. Most pieces of lava rock are dark in color and are frequently used in aquariums that house African cichlids. Small pieces of lava rock are frequently used in the biological filter compartment of aquariums.

    Quartz

    • Quartz is an abundant mineral and available in a number of different colors, including rose, smoky and citrine. Quartz is frequently available in attractive shapes and is found in the aquariums of many hobbyists. Quartz is inert in your aquarium, as it is composed mainly of silica, which will not alter water chemistry. Rose quartz, which is found in Brazil, is a delicate pink color and looks very attractive under water. This mineral is not terribly heavy and can be used as a solitary accent feature in the aquarium, or a number pieces can be stacked to create a cave or back wall.