The marine aquarium hobbyist is faced with a wide variety of substrates to be used in the aquarium. Trying to decide what to use is usually based on aesthetics and function. In the past, the options were dolomite (calcium-magnesium-carbonate) rock or crushed sea shells/coral. While esthetically pleasing, the crushed shells can be more difficult to keep clean in the aquarium due to their varying particle size and difficulty of vacuuming to remove debris. For this reason, most applications of crushed shells will require a very shallow substrate bed of less than ¾”.
Dolomite is rather uniform with average particle size of 2-4 mm, allowing it to be easily vacuumed for cleaning and to be used in deeper beds of several inches. It would be fine for burrowing fish, but not for sand shifting fish.
Aragonite (a crystal form of calcium carbonate) can be found is sizes from 0.1-5.0 mm. It is also found in mixes with bits of crushed shell and larger particles to give a “natural” sandy beach lot to the substrate. The finer particles may be too fine for burrowers but are safe for sand shifters. The mixed size substrates will generally work well for all types of saltwater fish and invertebrates.
Oolite (multi-layer, spherical calcium carbonate) is the finest of the substrates, ranging from 0.5-1 mm in size. Its smooth structure makes it great for sand shifters, but not applicable for burrowers.
These substrates can be found as a dry form or as a “live” form substrate. The dry form is substrate that was collected, washed and shifted to give a more uniform size, and then bagged for sale. It will usually produce a fair amount of dust when first placed in the aquarium, so it is best to pour the sand into a bucket of water before placing it in the aquarium. Then, slowly add water to the aquarium. You will still have a bit of a dust storm for the first week or so until the finer particles settle out or are trapped in the filter system. The live form will be sold moist and has been colonized with millions of beneficial bacteria to help establish the biological filtration of the marine aquarium. Because of its moist nature, the live substrates will also be much less dusty when first placed in the aquarium. You will need water already mixed to the proper salinity levels and the proper temperature to fill the aquarium to avoid killing off the beneficial bacteria in the live substrate.
Besides providing a safe substrate for sand shifting animals or for burrowing critters, the substrate can also determine the application of a particular style to the use in a plenum, deep bed or refugium. A plenum system will require a substrate with a larger size (1.0+ mm), while a deep bed will require smaller grain size (0.1-1.0 mm). A refugium would benefit from a mix of sizes. The calcium based substrates will also offer some buffering in the marine aquarium, preventing a drop in the pH level.