Substrate for a Saltwater Aquarium

Aquarium fishes, Angelfish (Pterophyllum altum)
DEA - C. DANI/De Agostini Picture Library/Getty Images

Much more than just the "stuff on the bottom of the tank", the aquarium substrate is an integral part of any saltwater aquarium system. Besides contributing to the visual appeal of a tank, substrate mediums also serve other purposes:

  • The substrate in a marine aquarium is a vital part of the biological filter base of an aquarium, and if you choose to use a non-living media over live sand, it will become " alive" as the tank goes through the cycling process as the aquarium matures.
  • Fine-grained mediums (1.0 to 2.0 mm diameter grain size) are used for NNR (Natural Nitrate Reduction) in conjunction with denitrification filtration setups, such as plenum, fluidized and deep sand bed filters, while courser grain sizes (2.0 to 5.5mm) may be used in wet/dry trickle filters, or when a UGF (Undergravel Filter) is present.
  • Fine-grained mediums (0.5-1.7 mm grain size or sugar-sized sand) is best for sand sifting fish such as Gobies.
  • It provides refuge for many types of bottom-dwelling marine fish and invertebrates that either build burrows or bury themselves in the substrate, like various species of Wrasses, Gobies, Blennies, Mandarinfishes, Shrimps, Starfishes and such. A somewhat fine to small grained medium is good for these types of animals as the heavier substrates may damage the animals as they plow through it. Wrasses, in particular, seem to incur external infections when kept in the heavier substrates.​
  • Certain types of sand-sifting invertebrates ingest substrate and extract organic matter from it, like sea cucumbers. Animals such as these rely on the substrate as a source of nutrition, and therefore a rather fine-grained medium should be provided.
  • The substrate houses other smaller critters, such as amphipods, copepods, and marine worms, which are a source of food for some other marine animals.
  • Natural ocean mediums are calcium-based, which is beneficial to saltwater systems in many ways.
  • A calcium-based media of the right consistency makes a good material for use in a calcium reactor.
  • It is a material used for making DIY rocks.

There can be quite a price difference between different grades and brands of substrates.

When deciding on what "consistency" of substrate to buy, as you can see one has to not only consider what it is going to look like, but factor in what it is going to be used for, if it will be appropriate for the kind of system you are setting up, and conducive to the particular types of marine life that will be kept in it.

Before you purchase your substrate material, keep in mind that heavier substrates have a tendency to stay put on the bottom of your tank. If you are going to have a number of powerheads in your tank for a lot of water circulation, you will want a heavier substrate.

One last thing to consider is "where" the medium comes from. Sand (like the type used in a children's play sand-box), gravel (such as the kind found in fish stores that comes in pretty colors that is primarily sold for freshwater tanks), as well as rocks that are quarried on land often contain unwanted minerals, metals, silicates, and possibly chemicals or toxins that may leach out into the aquarium water and become problematic. Therefore, land-based materials are not recommended.

The best kind of substrate to choose is one that is naturally high in calcium content that originates from the ocean. Aragonite, coral/reef, and shell-based mediums are the top choices, which can be bought in various colors such as pink, black, white and neutral tones, and come in a wide range of consistencies from very fine to small grain, and medium to coarse textures.

  • Tip: For aquariums where no "sand" sifting or dwelling animals are present, a somewhat large or coarse-textured shell media can be very eye appealing, such as in a specific fish-only predatory tank community that houses Triggers, Lionfishes, Puffers, Hawkfishes, Eels and such.