MimicTang,
Would support what others have written, but would support it with passages from two books by authors more experienced than I.
Calfo & Fenner write
"Sugar Fine grade (0.2-1.0mm) ...This is the ideal grain size for most detritivores and sand-sifting reef animals."
Further
"Medium grade sand (1.0-2.0mm) is similar in form and function to sugar-fine media. [But] this sand is borderline inappropriate (too large) for sand sifting detritivores like sea cucumbers and gobies."
Reef Invertebrates, Calfo & Fenner, 2003. p.34
In Nilson and Fossa's "Reef Secrets" it is stated that depth, rather than an actual debate on grain size for detritivores is the more important aspect of the sand bed. Apparently the authors assume the finest grade of sand available is employed with a depth of " up to 3 to 5 cm thick can be kept sufficiently stirred and oxygenated by water movement alone."
Further,
"If you keep animals that dig much into the substrate, such as sand-sifting fishes, burrowing crustaceans, or sea cucumbers, the bottom layer may be 2 to 3 times as thick without fear of having anaerobic patches develop." (p. 61)
And specific species have specific requirements for substrate, or even mixtures of substrate. One example is the prawn gobie (Amblyeleotris spp.) and its commensal the alpheid shrimp, that require a "deep bed of fine sand and bits of coral rubble."(p. 60)
In the stocking guide of the same volume a distinction is made between the Feather dusters Megalomma sp. and Bispira sp. for example. While the latter has a smooth tube and no specific requirement for larger sized substrate, the former uses larger sized sand grains in the formation of "the protective tube." (pp. 136, 137)
HTH
