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.
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How much and what to dose?
I’d suggest dosing sodium silicate solution, as it is a readily soluble form of silica. It is very inexpensive. I initially used a high quality laboratory grade, but I’d expect the bulk grades sold to the world at large to be good enough (and I use it now). Remember, you aren’t dosing much, and the solutions available are very concentrated. You may find “water glass’ in certain stores because it is used by consumers for things like preserving eggs. Buying chemicals can be problematic for many people, however, and this hobby chemistry store sells to individuals. Ten dollars (+ shipping) gets you enough to last 150 years of dosing with a 100-gallon tank, so cost is not an issue. I just ordered some from them myself and it came broken open, unfortunately. Some of you may have gotten Christmas presents that had ¾ of a gallon of sodium silicate solution coating them as they passed my package in the mail. Nevertheless, I
still have enough for several years!).
Many “water glass” or sodium silicate solutions are sold with the concentration indicated by “° Baume”. Degrees Baume is a measure of the specific gravity, and values in the 40’s are typical of these concentrated solutions. A concentration of 41° Baume equates to 29% SiO2 by weight. Note that the density is high (1.38 g/mL for 41° Baume), so volume measurements should take this into account. Maybe eventually, some of the hobby supplement manufacturers will provide a supplement.
Safety note: Sodium Silicate solution is very basic (high pH). In fact, the pH can be substantially higher than limewater, so it is very corrosive to tissue and to metal devices. Be careful to not spill it on yourself, wear some eye protection, and if you spill it on something metal, wash it. In all cases, extensive washing with water is recommended in case of spills or exposure.
Based on my dosing experience, aquarists are probably safe dosing the equivalent of 17 uM (1 ppm SiO2) once every 1-2 weeks. That is based on the fact that my tank used that much in less than 4 days without having any sort of “bad” reaction. Of course, there’s nothing wrong with starting at a tenth of that and ramping up. And, of course, if you do get too much in the way of diatoms, just back off on the dosing. I presume that all that I added to my tank went into various organisms that us it (sponges, diatoms, etc), but perhaps I have more sponges than other aquarists, and diatoms consequently may be more of a concern in some tanks than in mine.
I’d also advise occasionally checking the soluble silica concentration in the water, in case the demand in your tank is substantially less than mine. If the concentration started to rise above 50 uM (3 ppm SiO2), even in the absence of diatoms, I’d probably reduce the dose rate because that is close to the maximum concentration that surface seawater ever attains.
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