Silica based sand is fine for an aquarium. Silicates will not leech into the system from silica based sand. Theoretically, aragonite based sands can help with buffering, but the dissolution of aragonite in water is ph dependent. For there to be significant buffering from the aragonite, you'd have to have a VERY low tank ph, far too low for the survival of many inhabitants. Kind of like turning your tank into a giant calcium reactor.