In all of my discussions about sand beds I have made a point of specifying one particular parameter, that of the average size of sediment particles in the sand bed. Why should this one factor be so important? The answer simply is that sediment particle sizes determine the acceptability of the sediment to the organisms. Perhaps an example might illustrate this statement better. One of the common amphipods found along the west coast of North America is a species called Rhepoxynius abronius . This small bug has been investigated in some detail as an organism to use to test the toxicity of sediments, has been found to prefer sediments of a specific particle size, 0.113 mm in diameter. If given a choice, it will move to and live in sediments of that one specific size, not sediments 0.110 mm nor sediments of 0.115 mm, but only of that one size. If individuals are experimentally confined to other sediment sizes, they neither live as long, nor reproduce as well, nor tolerate stressful conditions as do individuals kept at the optimum grain size (Ott, 1986).
Most sediment-dwelling organisms appear to have similar precise preferences. However, most will also live at least marginally well in mixed-sediments with sizes around their optima, and most sediment particle size optima seem to be in the range of 0.050 to 0.200. Consequently I suggest a range averaging about 0.125 as a good compromise. It isn't specifically the best for most infaunal species, but it will allow a diversity of species to live pretty well.
Coarser sediments such as gravel or crushed coral are simply too big. Additionally, they have the drawback of being sharp edges that are abrasive to many of the small crustaceans and worms that must crawl through the sediments. Finer sediments can pack so tightly together that they are impervious to most animal movement, creating a layer that restricts animal and water flow shutting down the biological filter.
Having to assess sands for particle sizes would be a daunting task for any hobbyist. Fortunately, however, several vendors sell bulk sands in the appropriate size ranges, often marketed as "sugar fine" or oolitic sands. A few larger particles in the sediment mix is okay, but larger sediments should not constitute more than about 15 percent of the total. Under NO circumstances should you use crushed coral or coral gravel. These substrates are too coarse and often too abrasive for many of the smaller organisms to survive in.
