How does Silicate and Silicic Acid get into your Aquarium
Many water sources contain silicates or silicic acid, or compounds that contain these elements and these compounds eventually break down, adding silicates and silicic acid to your aquarium's water.
In aquariums where sandy or other forms of substrate are used, silicate can leach "out" of these compounds, and enter the tank's water very quickly. It usually only takes a few weeks for this to happen (at the maximum). Often it happens much sooner. The latter is especially the case if silica sand is used as the substrate in the aquarium (this is too often the case,unfortunately). Although less common now than a year or so ago, hobbyists were adding "play sand" to their tanks to build up a substrate. Play sand and so-called blasting sand are very high in silicate and should be avoided if you wish to keep silicates low.
My point is that silica sand is not going to be laboratory grade quartz with low soulubility, the cheap play sand is going to contain high amounts of soluble silica which leeches into the water at high phs. If its 99% quartz sand the soulubility will be about .03% (wich is lower that your aquarium glass and silicone seals on your aquarium), no problem. But a lower quality sand could contain a significant amount of feldspar (or any other particularly soluble silica compound) contamination, silicates leech into water from opaline deposits and such found in regular aragonite sand, why do you think there is a diatom bloom upon starting a new aquarium? because the soluble silica contained in your glass and silicone seals leaches into your water plus whatever opaline deposits are in your sand.
Three common forms of silica found in water are soluble (dissolved), colloidal and particulate. Soluble silica is commonly referred to as reactive silica, while colloidal silica may be called non-reactive or polymeric silica. The particulate kind may also be as suspended or granular. Colloidal silica contaminants are often combined with organic material. One problem high-purity water end users face in the removal of silica is that colloidal silica, unlike reactive silica, cannot be measured with conventional on-line techniques.
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