I'd aim for about 1 ppm. Water glass is suitable, as detailed in the article below.
Here's the section in diatoms from my silica article:
https://www.advancedaquarist.com/2003/1/aafeature1
With one exception (discussed below) all diatoms require silica for growth, and low silica levels cause significant changes in the cell cycle.7 Silicon is a major limiting nutrient for diatom growth in certain parts of the oceans,8 although
iron,9 nitrogen, and phosphorus can also be limiting. There have been many studies on the uptake of silica by diatoms. Most diatoms take up silica in the form of silicic acid, although one has been shown to take up the silicate form.10 If absorbing silica is a limiting factor, then it makes sense to transport silicic acid since it is present at much higher concentrations than is silicate, and hence is potentially easier to transport.
Different diatom species have different abilities to absorb silica from the water. That is, as the silica concentration drops, some diatoms can continue to pull silica from the water while others cannot. Most diatoms have half maximal rates of silica absorption of 0.7-10 uM (0.04 – 0.6 ppm SiO2),2 but some are substantially higher, up to about 60 uM (2.6 ppm SiO2).2 The in-situ average for biogenic silica uptake in the surface layer of the equatorial pacific showed half maximal uptake at a silica concentration of 1.6 uM at 3°S and 2.4 uM at the equator, which was close to the silica concentrations present.3
There apparently are genes for many different silica transporters in each of the diatom species that has been investigated.8 Diatoms also somehow maintain internal silicic acid concentrations at levels higher than its solubility, but the mechanism for accomplishing this is unclear. Nevertheless, it is obvious that this facilitates the deposition process, and inhibits dissolution of the existing frustule. Diatoms apparently use proteins to guide the deposition process, where soluble silica is converted into the intricate solid frustule, but exactly how this role is accomplished is not known.8
In a reef tank like mine with silica concentrations below 0.8 uM (0.05 ppm SiO2, the practical limit of the Hach silica kit), some diatoms will have a hard time absorbing silica. Many reef tanks may, in fact, be selecting for diatoms that are able to get enough silica at the low concentrations typically available. Are diatoms silica-limited in reef tanks? That question is addressed experimentally below.
In the oceans, diatoms are silica limited in some natural settings (like the polar regions and the Sargasso Sea, where the ambient silica concentration is less than 1 uM (0.06 ppm SiO2).11 There have also been many cases where eutrophication of natural waters has raised nitrogen and phosphorus levels to the point where silica has become limiting,12 even when it was not limiting in pristine waters. In reef tanks, where nitrogen and phosphorus are often not in short supply, it makes sense that silica could be limiting. In case you were thinking that silica limitation to diatom growth is necessarily a good thing, there are drawbacks. The limitation of silica, inhibiting the growth of diatoms that would otherwise take up the limiting nutrients nitrogen and phosphorus, has even been implicated in blooms of cyanobacteria.1
