Substrate is often used for cosmetic reasons to create a natural looking environment inside the aquarium. But in addition to appearance, aquarium substrate plays a vital role in biological filtration by providing surface area for nitrifying bacteria to grow.
Rock, whether it is live rock or dry rock, is used to "aquascape" your aquarium. It can be used in fish only systems (many times referred to using the acronym FOWLR, which stands for Fish Only With Live Rock) and is a must for reef systems.
Like sand, the rock in your saltwater aquarium will become biologically active to help reduce ammonia, nitrite and nitrate in your tank. Live rock already has this bacteria in it where dry rock will need to be seeded with bacteria to kick start the nitrogen cycle.
Sand comes in many different grain sizes. Larger grain sizes—often referred to as crushed coral—are frequently used in fish-only aquariums because they allow for easier removal of detritus by simply vacuuming the substrate. In a reef aquarium, finer grain sands are often used since the substrate provides a larger surface area and can play home to an abundance of marine life.
Sand is available as dry or live sand. CaribSea Arag-Alive contains nitrifying bacteria that 'seeds' an aquarium to speed up the cycling process. For fine substrate, 90 pounds covers a cubic foot. For coarser grains, 70 pounds covers a cubic foot.
If you need help deciding how much substrate you need, you can use our sand bed calculator to help determine the correct amount.