Live rock
Limestone rocks colonised by bacteria, sponges, microorganisms and small invertebrates. In marine aquariums they are the main substrate for biological filtration.
Detailed explanation
Live rock is the foundation of any marine aquarium. They are not simple decorations: they are miniature ecosystems. Every kilogram of good-quality live rock contains millions of nitrifying and denitrifying bacteria, sponges, worms, copepods and other microorganisms.
The recommended quantity varies but a historical rule says 1 kg for every 5 litres of water. Modern aquariums tend to use less rock to leave more space for swimming and improve circulation.
Natural live rock comes from coral reefs and their collection is increasingly limited for environmental reasons. Man-made alternatives such as MarcoRocks, Real Reef Rock and ceramic rocks are now widespread. They do not start with the same biodiversity as natural ones but they colonise within a few months.
The arrangement of the rocks (aquascaping) is not just aesthetic. It is necessary to create stable structures with arches and cavities that allow good water circulation. Dead zones where water stagnates become hotbeds of cyanobacteria.
Dry rock is often used as an economic base. They are slowly colonised by bacteria. Adding a few pieces of real live rock speeds up the colonisation process.
Limestone rocks in freshwater aquariums are only useful for aquariums with high pH (African cichlids). For most tropical freshwater aquariums, inert rocks such as basalt or lava rock are preferred.