Like many other people, I have pretty fast diatom growth on the glass of my aquarium, and have to scrape a lot. I have seen a number of solutions proposed by various people about controlling diatom growth in tanks, and the general consensus is that diatoms are regulated by silicon in the tank water. In the current Salt Solutions, there is a response to a similar question that says the principle source of silicon in a marine aquarium is quartz gravel which should only ever be used in freshwater tanks. I have crushed coral in the bottom of my tank, and so that can’t be my problem, but I still have a lot of diatom growth. The other recommendation was to use a high quality salt mix, which I do, and I recently tested the silicon level in my tank and the kit read zero. Why do I still have so many diatoms growing in my tank? The part I really don’t understand is why they are only growing on the glass of my aquarium, and I actually don’t ever see them anywhere else in the aquarium. Can you please tell me how I can control these pests?
There are several issues that are raised in your question here: 1) are diatoms are really deserving of their reputation a pest or not? 2) Does silica content of saltwater generally control diatom growth? 3) Is quartz sand a major contributor to soluble silicon content of aquarium water? And 4) Why do diatoms seem to always grow fastest on the glass walls of the aquarium. Well, that is quite a lot of ground to cover here, so please bear with me if it seems to be a choppy response – I’ll try to get to all of it. The first statement I would make is that I think diatoms have an undeserved bad reputation in the aquarium hobby. I’ll try to explain this a little more as we go through this answer, but let me just start by saying that I don’t think that the presence of a reasonable growth of diatoms in your system is anything to be worried about, and perhaps is even something to be encouraged. Furthermore, even a professional marine biologist cannot tell you whether or not that brown film in your aquarium is the result of diatoms or not without looking at them under a microscope, and I suspect that the bad reputation of nuisance “diatoms”in reef tanks is largely the result of misidentifications of other trouble-makers, such as dinoflagellates.
That aside, even assuming that the brown film in your tank is indeed diatoms (as I would guess they are by the fact that they grow primarily on the walls of your aquarium), you say that the general consensus seems to be that diatoms are limited by the concentration of silicon in tank water. I would say that is perhaps true when looking through the aquarium literature, but would argue that a different picture emerges when looking at nature. Silicate is used by diatoms in the formation of their frustules (a glass-like “shell” in which the diatom lives), and it is true that in some cases addition of silicate to an aquarium can result in diatoms getting a competitive edge over other algae. That is not the whole story, however. I’m sure that if I asked you what fuels the growth of hair algae or some other “pest” species, you would tell me nitrogen and phosphorous, and that is why reef aquarists strive to keep the concentration of compounds such as ammonia, nitrite, nitrate and phosphate at low levels in their aquaria. Well, the fact is that diatoms are no different from any other species of unsightly and unwelcome algae in an aquarium – they need nitrogen and phosphorous just like hair or slime algae do; they just need silicate in addition to those other compounds. The concentration of silicate by itself has no bearing on whether or not you’re likely to have an algal growth problem in an aquarium, it just determines whether or not diatoms are able to be part of the algal community that is using the available nutrients or not. The fact is that in most studies of diatom growth in nature, silicate was found not to be the limiting nutrient, and with a few exceptions (such as the Sargasso Sea and Antarctica) diatoms blooms in the wild are thought to be only rarely limited by the availability of silicon.
Let me try to summarize a complex and prolific scientific literature on the subject: There are seasonal changes in natural communities that result from changes in the ambient lighting, temperature, salinity, water-column stability, nutrient availability and the ability of different groups of algae to exploit the prevailing conditions. In general, summer communities (typically dominated by dinoflagellates, which are most commonly mistaken for diatoms) have higher light optima, shorter generation times (and consequently increased doubling rates) are motile and have a competitive edge at decreased nutrient concentrations. Winter communities (typically dominated by diatoms), on the other hand, have lower light optima, longer generation times (and consequently decreased doubling rates), a greater capacity for energy storage (hence the reason that they make such excellent and necessary food items for a wide variety of grazing animals), and have a competitive edge at increased nutrient concentrations. If the concentration of phosphorus and nitrogen are maintained at low levels, diatoms should always lose out in competition to dinoflagellates regardless of the ambient silicate concentration, and I would argue that is the state of the majority of reef aquaria. Aside from the fact that dinoflagellates grow much more quickly, they are also typically chemically defended, and some species can be downright poisonous (such as Gymnodinium or Alexandrium, the dinoflagellate species which are responsible for phenomena known as Red Tides). Of course, that is not to say that all diatoms are without defense. Some diatoms (such Chaetoceros or Pseudonitzschia) can have chemical or physical defenses to avoid being eaten as well, but the majority of diatoms are substantially less defended and more commonly consumed by marine animals than are the dinoflagellates.
Craig Bingman recently reviewed the amount of silica in artificial salt mixes, and wrote an excellent article on silicon and aquarium husbandry that suggests that it is not only good, but ought to be recommended that you maintain enough silicate in a reef system such that diatom growth was high enough to necessitate cleaning the glass every day or two. I’d have to say that I agree that this is a sign of a healthy system, and that diatoms are an important part of marine food chains to maintain in any reef aquarium. Rather than being upset that you need to clean your glass daily, you should be happy that for the effort of regularly scraping your glass, you have a highly nutritious unicellular organism in your tank that not only competes with less desirable species (such as hair and slime algae) for nutrients, but packages those nutrients in ideal little bundles that are an important source of food for a lot of grazing and suspension feeding animals on the reef.
The next point that you bring up is widely believed and often repeated, but unfortunately just wrong. The principle source of silicate in any marine system is input of silicic acid from freshwater additions, not from the dissolution of quartz sand. Estimates of the silicon budget of the world’s oceans suggest that 80% of the silicon in the sea comes from freshwater run-off (primarily by river input), while the other 20% comes from the dissolution of old diatom frustules in the deep sea which are brought to the surface again in areas of upwelling (this is an area where currents result in pulling deep, cold, nutrient-rich seawater to the surface again), such as the west coast of North and South America. Quartz sand does not enter into the equation at all. In fact, quartz (silicon-dioxide, SiO2) is the least soluble of all forms of silicon found in nature. Although it is important to keep in mind that water is a universal solvent and everything dissolves in water given enough time, the walls of your aquarium are far more soluble than any layer of quartz sand you could place on the bottom of it. This is not to say that it is not possible to introduce silicate into your tank by the inclusion of silica sand – there are many more soluble silicate compounds (such as feldspar) or contaminants (such as aluminosilicate) which could be found in a generic bag of silica sand from the hardware store that could contribute to an increased level of dissolved silicates in an aquarium, but pure quartz sand is not one of them. If you are worried about having an increased silicate level in your aquarium, it is easy to test whether the silica sand you have chosen releases any silicates by adding it to a small volume of DI/RO water and letting it stand for a week or two before using a standard aquarium silicon concentration test kit.
Glass is really a super-cooled liquid that has become viscous enough that it appears solid to us. Aquarium glass is not simply melted quartz, but rather a mixture which typically includes sodium oxide and calcium oxide dissolved in the melt to lower the softening point and make the glass easier to work with. The addition of these other compounds which make glass easier to work with also make it more soluble in water. That is the reason that you are likely to see diatoms on primarily or only the walls of your glass aquarium – diatoms have a much easier time pulling silicate out of glass aquarium walls than out of quartz sand grains. I’m not saying that they can’t do it, but quartz is the hardest form of silicon in nature for diatoms (or sponges, protozoans, or other silica-loving animals) to use in the formation of their skeletons and is generally used only as a last resort. Given that bit of data, it's pretty hard to argue that using quartz sand is bad when the glass box that you're putting it into turns out to a better source of silicate for diatom growth.
In fact, the first published report of using “live sand” in an aquarium seems to be the aquarium system of Caswell Grave, who set up a tank in 1900 based on silica sand he dredged up from the sea floor. Using the sandbed as the basis for filtration and nutrition in his tank, he succeeded in culturing and raising sand dollars through metamorphosis and three months of healthy post-metamorphic growth (which he submitted in 1901 and was published in 1902). Caswell didn't have anything other than a window in front of which to set the tank, a supply of seawater and a few inches of live sand which he dumped onto the bottom of his aquarium, but he succeeded beyond that of most hobbyists with high-tech systems today.
So, just to sum up. First of all, I think that diatoms have a largely undeserved reputation as pests, and that moderate growth of diatoms in a marine aquarium is not only an indication of health, but in many cases actually beneficial to the system. Second, their growth is not usually silica limited in nature, although that may well be the case in marine aquaria where the concentration of nutrients is often skewed from natural proportions. Still, having said that, diatoms need the exact same nutrients as other less desirable algae, plus silicon, so if you don’t have diatoms growing, you may end up having something else that is harder to control with a few herbivores (most of the common “clean-up” animals available for the hobby specialize on, or at least prefer diatoms to other film algae when given a choice). Third, the standard dogma that silica sand leads to increased dissolved silicate levels may be true of feldspar or other nonquartz sands, but the fact is that quartz is a far less soluble source of silicon than the walls of your aquarium, and does not deserve the reputation that it has, either.
I want to thank Craig Bingman and Randy Holmes-Farley for their input on this subject during our discussions. Anyhow, I hope that answers your questions despite the fact that I did not provide any specific recipe for how to eliminate diatoms from your aquarium...