Portion of the Periodic Table of Elements emphasizing the Silicon element

Silicon (Si) is one of the most abundant elements in the earth’s crust. It's present in many minerals like quartz sand, granite, and sandstone. The natural weathering of these minerals release silicon into the water supply. Rivers carry a lot of silica into the ocean, which eventually makes its way to the reef. Although it is naturally present in reefs and many animals use it, aquarists have been concerned with the soluble forms (silicic acid Si(OH)4 and silicate Si(OH)3O) of silicate in the reef aquarium because it has long been believed to contribute to diatom growth, an unsightly nuisance.  

While it is true, diatoms are pesky, the facts reveal that silicate is not a bad guy at all.  In fact, it's quite the opposite - playing an integral role in the ecosystem and the minimal amount of silicate that is available depletes rapidly. Some evidence even suggests silica will help in the fight against far more severe algae problems, such as turf and hair algae. Let's take a closer look.  

What’s silica used for?

Silica is an essential element for many plants and animals. Plants use silica to strengthen tissues. Marine sponges incorporate silica into their structure, giving it strength and rigidity. Many members of algae clean-up crews, like Astrea snails, conchs and chitons have tiny teeth-like scrapers, called a radula, and one of the elements used to form these alga-scraping teeth is silicon! The controversial Diatoms extract silicate from the water to build their intricate shells.

This results in the rapid use of silicates in an aquarium and is thought to have some benefits. Being as our aquariums are most often "silicate limited" could the presence of silicate be more beneficial than harmful.  Since diatoms also need nitrate and phosphate along with silicate to grow, does this help compete with more harmful or pesky algae and dinoflagellates?  Diatoms also produce oxygen, consume CO2 and serve as a viable food source for zooplankton. Randy Holmes-Farley experimented with dosing silicates with surprisingly positive results.

Is silicate good or bad in a reef aquarium?

We can’t label silicate as a “bad” substance since it is needed by most marine life. However, many reef aquarists have connected excess diatom growth with elevated or high silicate levels in the water. When in excess, diatoms are a golden-brown "dust-like" coating on aquarium glass, substrate, and live rock which, for the most part, is simply unsightly. This is much more common in new aquariums and often just runs its course without causing too much of a problem. A new tank will just go through a diatom phase that generally goes away within just a couple of weeks after the silicates have been depleted. 

Diatoms cannot exist without silicate and use it incredibly fast along with other organisms in your tank. So if your tank is new and you're dealing with diatoms, they will likely go away within a short time so long as you monitor the sources of silicate going into your tank.  

Silicate Test Kit

Testing for silicate

Silicate test kits are designed to detect soluble silicate in aquarium water. You can test your tank, tap water, and RO water for soluable silicate to find out if your tank has elevated silicates and establish the source.  

If you are using a mail-in ICP test, be aware that ICP tests instead for silicon, not soluble silicates alone like an aquarium test kit. These results will include silicon from microscopic silicon-containing sand grains suspended in the water sample. You’ll need a silicate test kit to know the actual soluble silica levels in your water.

While silica is not yet 100% fully understood in the aquarium, it will most certainly be depleted quite quickly in an established tank without any substantial sources of elevated silicate. If you're going through a diatom bloom, lowering or limiting that silicate will help reduce the growth. If all is well (diatom free) and silicates are testing within range, nothing is wrong or needs fixing.  

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How do silicates get into the aquarium?

Diatoms and elevated silicates are common in new aquariums because of new aquarium sand, salt mix, and tap water. All three are the most common sources of elevated silicates. Fish food and some additives can contribute to silicate levels too but the amount is often neglectable compared to how fast the tank's organisms are using it.  

It should be noted that silicate is purposefully added to many municpal tap water supplies and using an RO/DI system will help remove this silicate before the water is used

Removing silicates from tap water

RO/DI system will remove silicates from the water using the TFC membrane and de-ionization cartridges
Bulk Reef Supply RO/DI Systems

The most efficient way to gain control of silicate in tap water is with a reverse osmosis (RO) filtration system.  Many reef-keepers already use RO/DI water to mix saltwater and freshwater top-offs. But not all RO systems are created equal and if you have detected high silica in your tap water, be sure your RO/DI system can handle it.

Certain substances in tap water, like silicate, can make it past the RO membrane. There are special TFC membranes designed to remove silicates at a higher rate than regular TFC membranes. Regardless, membranes only remove a percentage of the silicate. It may be removing 90% of the silicate, but if you want to remove more you’ll need a little extra help from the DI resin stage. The last stage in your RO/DI filtration system should be a deionization (DI) cartridge because DI resin cartridges capture the silicate that slips through the RO membrane, ensuring your water is ultra-pure.

Removing silicates in reef aquariums

Media like GFO can help gain control of phosphate and silicate

If your reef tank is having a diatom algae problem, test for silicates to get a baseline level. Phosphate-removing filtration media like GFO and activated alumina will also remove silicates. If you have high phosphates and silicates in the aquarium, the rate of silicate removal will be reduced until the high phosphate is under control.

Recommended silicate levels for reef aquariums

If you’re looking for a specific “safe” level of silicate you won’t find it. Diatoms need all the essential nutrients to grow. You could have plenty of silicates and no diatom growth, but maybe because another essential nutrient is low and inhibiting the algae. Some aquarists like to keep the silicate level below detection on a test kit. Others simply watch for diatom growth. If the diatoms start to grow, test the water and use a phosphate remover to bring the silicate level down a bit.

Download our Reef Tank Parameters chart to learn more about aquarium water parameters.

If you want to gain control of your silicate level, the first place to start is with the water you’re adding to the tank. Make sure your RO is “silicate-ready” with the proper TFC membrane and DI cartridge. This will make it much easier to keep diatoms and silicates under control. If your tank already has a diatom problem, use a phosphate remover to reduce the silicate level to where the diatom growth is under control.