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[wildeone]

And, if that didn't bore you enough, here is an article on Silica Sand (Quartz) for reef use. From http://www.reefs.org/library/article/r_toonen9.html

Silica Sand

Rob Toonen

Posted to Reefkeepers emailing list, Saturday 18th September 1999.

I can't remember who said it anymore, and I don't really want to single anyone out, but the statement that buying cheap sand that contains quartz will ruin your tank is plain-and-simple bunk. I'm not sure where the idea that silica sand is dangerous to a reef tank came from, but typically silica sand is 99.0-99.9% SiO2 (depending on the source and grade), which is about the exact same chemical composition as the glass of your aquarium. If the addition of pure quartz sand is somehow dangerous to keeping a reef tank, we'd better all get our animals out of glass aquaria...
Quartz (SiO2) is considered "totally insoluble" in water according to the US MSDS, and is also nontoxic (although inhalation of silica has many well-documented health risks for humans -- I'll explain at the bottom if anyone cares). Yes, water is a "universal solvent" and yes, everything (including the silicone) dissolves slightly into the water over time, but the amount of dissolution is so low that it is impossible for it to make a difference to your aquarium. There certainly are highly soluble forms of silica that will increase the level of dissolved silicates in the water (such as aluminosilicate) and are likely to cause problems, but quartz sand (SiO2) is not one of them.
The fact is that quartz sand (and the walls of our aquarium and even the silicone rubber which is the most soluble of the lot) do not dissolve enough in seawater to be measurable If silica sand contributed in any significant way to dissolved silica, then you would expect there to be big differences in the silica concentration around sandy beaches and on calcareous beaches (such as the red "sand" of Bermuda, which is composed mainly of calcareous foraminiferan skeletons) but there are not -- although calcium concentrations do vary significantly, the silica concentration in either location is about the same (roughly 2 ppm everywhere other than adjacent to the mouths of rivers where FW inputs increase the level). That suggests to me that quartz sand doesn't make much of a difference to the silicate concentration of seawater. Silica gets into water by being in a more soluble form than SiO2 (such as aluminosilicate), and the most common source of contaminating silicate in aquaria is the freshwater used for top-off or mixing. In fact, normal river runoff entering the sea has 2-5 times the amount of dissolved silica present in the surrounding seawater (which as I just said is higher than the norm), and researchers studying oceanic silica cycles consider quartz sediments a "dead end" for silica (so little is released it does not contribute to the global silica budgets of the ocean -- if it doesn't make a difference on a global scale with all the silica sand in the ocean, how much difference do you think it can make in our tanks?). The major input of silicate into seawater remains freshwater runoff into the sea, not the minuscule (and unmeasurable) amount of dissolution from the *enormous* amount of quartz in the sea...
In fact, that same quartz sand that people are recommending against is what was smolted and fused to form the glass walls of your aquarium... I don't know exactly what (if any) chemical changes are involved with the smolting process, but according to the glassblower for the Department of Chemistry, it's just melted and reformed into the appropriate shape -- it's not really doing anything to the sand other than burning off any organic contamination in the sand (the melting point is about 3110F). Even after being formed, glass is still SiO2, so there isn't any reason to suspect that there are important chemical changes occurring. Also, there should be no changes occurring as the sand passes through the guts of the animals in the tank -- is no noticeable degradation of the SiO2 spicules from ingested sponge tissue as that passes through the guts of animals (such as angelfish, sea stars & urchins) adapted to eating sponges, and that's where you'd expect some effect of digestion if any was going to occur. Given that, it's pretty hard to argue that using quartz sand is bad when the glass box that you're putting it into is made of the same stuff.
The standard recommendation to decrease silicates applies to your *water* not your sediments. If you look through D&S, Moe, Spotte, Adey and Ron's articles they all have a sentence buried somewhere that says something to the effect of the use of silica sand for sediments is fine, but that carbonate sands have a couple of advantages that make it more desirable (primarily the ability to absorp phosphate, and the buffering capacity, which I'll come back to later). Somewhere along the line, someone got it in their head that keeping silicates low in the aquarium meant not including quartz sand, because that is primarily silica. This "fact" has spread across the net, but it's another one of those things that "everyone knows" but that lacks any convincing evidence (or logic for that matter).
Having said that, however, some diatoms are apparently capable of pulling silica out of glass or quartz, and if when culturing phytoplankton in glass containers, additional silicates are usually not added to the culture medium because the algae seem to get what they need from the walls of the flask (hence to common observation that diatoms grow fastest on the walls of the aquarium). Even with the presence of lots of dissolved silicate in the tank, however you still need the other nutrients (nitrates and phosphates) to get a bloom of diatoms. In fact, diatoms turn out to need *more* nutrients in culture (people generally also add glutamate & glycerophosphate to culture media to really get diatoms growing well) than standard microalgae. If there are high silicates but undetectable levels of nitrates and phosphates, you're still unlikely to ever get a significant increase in diatom growth. Some people *do* report having diatom blooms with silica sand (although the cause for such blooms is unclear). Stories about "I used quartz beach sand and the diatoms exploded in my tank" may be coincidence, it may be that the diatoms were already there growing on the glass, but the addition of sand gives them more surface area to inhabit and therefore they seem to bloom, or it may be that by using the beach sand, those diatoms were *introduced* to the tank in the first place. I think that the "dreaded diatom bloom" is as likely to be attributed to the introduction of the diatoms as to the provision of nutrients...
In fact, I have a couple of tanks that are set up on washed silica sandbeds from the hardware store, and I don't see any problems with them. When I first started to get into using deep sandbeds in my tanks, there were *no* carbonate sands available through any sources, so I had no choice but to use silica sands to establish my early sandbed tanks. Those tanks are still set up and running just fine back in Canada despite the fact that my father (who has a black thumb of coral death) took them over when I left home many years ago . It makes no difference to the animals in the sandbed what the grains are made of, and silica is actually more resistant to size degradation over time than are carbonate sands (which continually get smaller due to dissolution and passage through the digestive system of animals in the tank). Of course, therein lies the benefit of using aragonitic sands -- the dissolution of these sediments allows for some increased buffering capacity and calcium release into the aquarium and the organisms which ingest them. That is a benefit in itself and a good reason to use carbonate sands if they are available. However the buffering capacity of various carbonate substrata is also overblown. Laboratory experiments comparing the buffering capacity of crushed oyster shell (composed primarily of the most soluble forms of calcium carbonate -- high magnesium calcite and aragonite), crushed coral gravel, and dolomite sediments compared to that of silica sand. Of course silica sand had no buffering capacity and the tank pH dropped below 7.0 within 90 days. The other sediments didn't work so well, either -- after two months in a tank with a reasonable bioload, the pH of seawater in the tank with dolomite was 7.4-7.5, the crushed coral/aragonite was a little better at 7.7-7.8, and the oyster shell was the best at 7.8-7.9 -- again though, this was over only 2 months. In any case, none of them buffered the pH enough to rely on the capacity of the sediments alone to provide buffering over the long-term.
I'm not saying that anyone should stop using carbonate sands, or that there aren't advantages to doing so. I'm simply trying to point out that this is another one of those great urban legends that runs around in the hobby. Once everyone "knows" it's true, no one seems to question it anymore, no matter how improbable the factoid seems. I certainly prefer to use oolitic sand and have in all my recent tanks, but I'm trying to explain that the sandbed critters couldn't care less for the most part (there are obviously some exceptions) as long as the particle size is correct, and that it is not *necessary* to do so.