Live rock is used in many modern saltwater tanks for several reasons. It is home to many different organisms, hence the name “Live Rock”, including the microscopic bacteria responsible for the nitrogen cycle. Live rock acts as a medium for many different beneficial systems that are essential to the health of the system, including the filtration. It also hosts a number of alga and crustaceans for the tank’s inhabitants to feed on.
Filtration provided through live rock is effective and simple, a tank with a sufficient (at least ¾ Lbs per gallon) live rock and sufficient water movement through water pumps will completely extinguish the need for a filter on the tank. Mind you, the live rock has no way of removing particles from the water column, so the addition of a small filter may be desirable specifically for this purpose.
This picture shows the growth that is often seen on live rock. The purple coloring is a type of calcareous algae called Coralline algae.
Live rock is a calcareous rock that is formed from the skeletons of corals, among a number of other organisms that sport calcareous skeletons, likes mollusks.
The main downside to live rock is that because it is mainly collected from the ocean, it is often home to a large number of organisms, some of which are very undesirable to have in your tank. Lice rock often brings in a great number of beneficial critters that help your tank in a number of ways. Unfortunately it only takes one of these bad hitchhikers to ruin the balance in a system.
In this picture, you can see how the live rock has the shape of a large hard coral colony, the small holes that protrude in some areas of the rock were once the corallites (areas where the polyp is attached to the skeleton) of the coral. On this particular piece you can see two different groups of hitchhikers. In this case, the hitchhikers were welcomed; they are the Brown and Green Zoanthid sp. corals.
Often fetching high prices even for the high prices even for the lower quality rock, it is still a desirable investment in a marine tank, especially a reef tank. There are a number of Do It Yourself recipes for creating your own live rock using a concrete, and sand mix. This allows you to avoid the bad hitchhikers and also allows you to create your own shapes. Using this DIY method does save lots of money, but it lacks the beneficial bacteria and other organisms that are needed for a stable system.