Any object or substance that contains calcium carbonate, or calcium, can be described as calcareous.
In freshwater aquariums some calcareous substances will raise the waters hardness, often to levels that are unsuitable for most fish. In marine (saltwater) aquariums, calcareous substances will help to buffer and stabilise water conditions and provide vital calcium for 'hard' corals to form their skeletons.
Many of the substrates and rock decorations designed for marine aquarium such as coral gravel (which may be made from long-dead coral deposits or crushed shells), ocean rock, and tufa rock, are calcareous and should not be used in the majority of freshwater aquariums.
Rock types such as chalk, limestone, and marble are also calcareous, and be wary of any rocks that have a holed or structured appearance; this is often a sign of uneven erosion caused by calcareous elements in the rock.
The only exception in which calcareous rocks and substrates may be used in freshwater aquariums are in cases where hardness and alkaline loving fish are kept, such as rift-lake cichlids. Calcareous rocks can be used in brackish aquaria.