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. Its 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, many aquarists are concerned with the soluble forms of silicate in the reef aquarium - silicic acid Si(OH)4 and silicate Si(OH)3O- - as they can contribute to algae problems. So, before it does get out of hand, here's what you need to know about silicate and how to gain control of it, if it gets out of hand.

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 radula. One of the elements used to form these alga-scraping teeth is silicon! Diatom algae also extract silicon from the water to build their intricate shells.

Is silicate good or bad in a reef aquarium?

Silicate is an essential element for many marine organisms. We can’t label silicate as a “bad” substance since it is needed by marine life. However, many reef aquarists have connected excess diatom growth with high silicate levels. In the aquarium diatom algae appear as a golden-brown coating on aquarium glass, substrate and live rock. Diatoms on the glass look messy and make it hard to see through to the reef. An algae clean-up crew can help by grazing on diatoms coating the rock and substrate.  But If diatoms are out of control, it is a good indication there may be a silicate problem.

Testing for silicate

Silicate Test Kit
Seachem Laboratories Multitest: Silicate Test Kit and Salifert Silicate Test Kit

Silicate test kits are designed to detect soluble silicate in water. You can test your tank, tap water and RO water for silicate. Be aware that ICP test results are for elemental silicon, not soluble silicates. The results will also 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.

How do silicates get into the aquarium?

Tap water can be a significant source of silicates. Most water supplies have a natural background level of silicate. Your municipal water company may also be adding sodium silicate as a corrosion preventative. Silicate levels in tap water can be 8 ppm or even higher depending on the source of the water. If you’re using unfiltered tap water in your tank, chances are your adding silicate with every top-off and every batch of saltwater.

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 an RO to make saltwater and for top-offs. But not all RO systems are up to the task. Certain substances, like silicate, make it past the RO membrane. Be sure your RO unit is using the latest TFC (thin film composite) membrane. Even better, there are special TFC membranes designed to remove silicates at a higher rate than regular TFC membranes. That’s the good news. But the truth is RO only removes 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 help. If silicate is a concern, the last stage in your RO filtration system should be a deionization (DI) cartridge. 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 algae start to grow, test the water and use a phosphate remover to bring the silicate level down a bit.

A chart of parameters which include silicates to help guide control over the elements and gain stability

Conclusion

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.