Silicates cause algae (diatoms), silica (sand) does nothing, it is not really soluable in a marine tank to any damaging degree. Aragonite sand can be tested with vinegar. Put a small amount of the sand in a glass bowl and a teaspoon or so of vinegar. If it fizz's you have aragonite, if it does nothing you have silica.
though, Quikrete playsand is silica.