@Spare time btw, is silica the same as silicates...as in the nutrient diatoms feed on?
Sort of.
Why would I recommend dosing silica? Largely because creatures in our tanks use it, the concentrations in our tanks (at least in mine) are below natural levels, and the sponges, mollusks, and diatoms may not be getting enough to thrive.
reefs.com
Silica in the Ocean
Dissolved silica in the ocean largely takes the form of silicic acid, Si(OH)4. Since it is acidic and has a pKa somewhat above normal seawater pH values (pKa ~ 9.5 in freshwater; possibly it is a bit lower in seawater), about 5% of it will be present as silicate, Si(OH)3O–. Apparently, many diatoms take up the Si(OH)4 form directly, although there is some evidence that certain organisms take up Si(OH)3O–. In this article, I will not generally refer to silicic acid or silicate unless I am specifying one or the other. Typically I will refer to the sum of them as “soluble silica” or just silica, if the context is clear.
The concentration of soluble silica in the ocean is highly variable. In near surface waters, diatoms are very efficient at sucking it out of solution to make their SiO2 frustules. A diatom bloom in the ocean can drive the concentration of silica down from a value not atypical for the whole ocean, 45 mM (2.7 ppm SiO2), to less than 1 mM (0.06 ppm SiO2), at which point the diatoms can become silicon limited.2 Typical silica concentrations in the surface waters of the equatorial Pacific are a few mM.3
Rivers are the primary input of silica into the oceans (80% of the total input; underwater vents and deposition from the atmosphere are also significant contributors), and river water worldwide averages 150 mM (9 ppm SiO2).4 Consequently, coastal areas near a river may have greater silica concentrations than open ocean areas. This input is approximately balanced by the deposition of silica on the ocean bottom. However, the total biogenic incorporation of silica into organisms is about 40 times as fast as river input, indicating that much of the silicon is deposited into skeletons and re- dissolved many times before it becomes “permanently” incorporated into sea floor sediments. The average residence time for a single silicon atom in the oceans is only about 400 years, before it gets deposited in some fashion.4
When diatoms and radiolarians die and sink, they slowly dissolve, and the silica concentration in deep water can be much higher than surface water for this reason. All ocean waters are undersaturated with respect to amorphous silica (allowing the silica structures in diatoms and radiolarians to dissolve), and most waters are undersaturated even with respect to quartz,5 although its dissolution is kinetically slow, allowing beaches to exist.
