03-23-2012, 07:22 PM
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it came recommended by presumably knowledgeable members of a couple of my plant forums
I hope it didn't seem like I was suggesting otherwise. Play sand is a good choice because it is crystalline silica: it is mostly insoluble in water at low temps and pressures and is chemically stable. The only issue is the inhalation hazard, just follow the manufacturers recommendations.
Silica is relevant with regard to diatoms because the cell walls of these organisms are made of silica, but it must be in a form they can utilize (which I believe is silicic acid). Crystalline quartz will not be a source of this since it will not dissolve in any significant amount in water at the temps encountered in aquaria - other more soluble forms of silica can, crystalline quartz won't.