There is nothing wrong with the silica sand if you can't find tropical playsand (aragonite). Any playsand would be suitable for a marine tank. Arogonite supposedly will buffer your pH. I have not found that to be the case.
"Several chemical properties make aragonite the ideal material from which to manufacture a high performance substrate:
1) Aragonite is metastable at a pH of 8.2 i.e. it begins to dissolve into its
constituent ions of Ca
++ and CO3
-- while calcite (typical "crushed coral")
begins to dissolve somewhere in the mid 7's.
2) Aragonite contains substantial amounts of the larger ionic radius +2 cations
such as strontium (up to 10,000 ppm
) which are released into the system as the substrate dissolves. Under laboratory conditions the presence of strontium in
solution appears to facilitate the inorganic precipitation of aragonite when
those solutions also contain sodium and magnesium (like sea water). This would
seem to confirm the observations of hobbyists who note an acceleration of coral
growth with the addition of strontium to a closed system.
3) Unlike calcite, aragonite continues to react chemically with sea water after
acquiring a bacterial biofilm coating. There are further ways to boost the
performance of aragonite. The physical properties of the material can be enhanced
by close grading which allows for more interstitial water per volume, and by
manipulating the surface area to volume ratio which states that if grain size is
uniform, the surface area as a function of volume increases as the grain size
diminishes. Finally, performance can also be increased by technological means
such as by a calcium reactor or the Eco-Sand plenum system. This combination of a
superior raw material boosted by mechanical and technical means is simple and
effective and lasts for the life of the system