Silicates in the Aquarium
What dissolved silica is, where it enters aquarium water, why it fuels diatoms, how reverse osmosis and anion-exchange resins remove it, and how the molybdate test measures it.
What "silicate" means in water
In aquarium usage, "silicate" refers to dissolved silica. In dilute water, silica is present mainly as orthosilicic acid, H₄SiO₄ or Si(OH)₄, a very weak acid. Wikipedia gives its first dissociation constant as pKa₁ = 9.84 at 25 °C, and a water-treatment patent describes silica as only about half ionized at pH 9.7. In the pH range of most freshwater and marine aquaria, dissolved silica is therefore largely un-ionized. When concentrations are high, silicic acid polymerises and colloidal silica forms; the Australian Drinking Water Guidelines distinguish this colloidal fraction from the soluble, "molybdate-reactive" fraction.
Results are reported either as silicon (Si) or as silica (SiO₂). The Australian guidelines equate 100 mg Si/L with 210 mg SiO₂/L, so a value expressed as SiO₂ is about 2.1 times larger than the same amount expressed as Si.
Where silicate comes from
Tap and well water
Source water is the main input. Silica dissolves from rocks and minerals, and the amount depends heavily on geology. Australian supplies monitored in 2019–2020 ranged from 0.6 to 90 mg SiO₂/L in Western Australia and from 11 to 104 mg SiO₂/L in the Northern Territory. A USGS survey of the Ogallala aquifer found up to 65 mg/L, unusually high for non-geothermal groundwater, and attributed it to amorphous silica in young volcanic-ash deposits. The Australian aesthetic guideline of 80 mg SiO₂/L exists to limit scale, not because silica is a known health hazard; the guidelines note little evidence of adverse effects in humans from silicon in drinking water.
Sand, glass and silicone
The form of the silica matters. The Australian guidelines give the solubility of crystalline silica as about 6 mg SiO₂/L, compared with 100–140 mg SiO₂/L for amorphous silica. Most inland and non-tropical sand is quartz, a crystalline form of silicon dioxide that resists weathering, whereas tropical coral and aragonite sands consist mostly of calcium carbonate. Contact with glass can add silica to water: the US EPA colorimetric silica method lists contamination from glassware as an interference and calls for silica-free reagents. Silicone sealant is chemically different from silicate: it is a siloxane polymer (a –Si–O– backbone carrying organic groups) with low chemical reactivity, and common acetoxy types release acetic acid while curing. No fetched source documents measurable silicate release from cured aquarium sealant.
The diatom connection
Diatoms are microalgae whose cell wall, the frustule, is made of hydrated silicon dioxide. They take up dissolved silicic acid as they grow, and the requirement for silicon is unique to diatoms among the major phytoplankton groups. In the ocean, silicon is often the first nutrient to run out, and the spring diatom bloom typically ends when silicon becomes scarce. Silica availability therefore directly controls how much diatom growth a body of water can support.
In aquaria, diatoms appear as a brown film over glass, substrate, equipment, plant leaves and decorations, typically in newly set-up tanks during their first weeks, and are usually replaced by green algae as the system matures. Removing the film by siphoning, grazing by snails, shrimp and suckermouth catfish, and lowering silicate in the replacement water are the measures generally described.
Removal: RO, DI and silicate-binding media
- Reverse osmosis: a US patent on high-efficiency RO states that conventional RO systems reject roughly 95–99% of silica. Silica is also a scaling risk for the membranes themselves.
- Deionization: only strong base anion (SBA) resins in the hydroxide form remove weak acids such as silicic and carbonic acid; weak base anion resins do not remove silica. Silica must depolymerise, dissolve and ionise before the resin can exchange it, which is why industrial regeneration of SBA resin uses longer contact time and elevated temperature.
- Combined RO followed by mixed-bed DI is the standard industrial route to high-purity water, with RO acting as pretreatment and DI as the polishing step.
"Silicate-removing" filter media sold for aquaria rely on the same chemistry that water treatment uses. A 2014 study in Desalination tested pre-removal of silica before RO: aluminium hydroxide removed nearly all molecularly dissolved silica (with some residual aluminium and aluminosilicate colloids), and an SBA exchange resin removed up to 94%. Iron hydroxide and silica gel were also tested as precipitants. Like any ion-exchange or adsorbent bed, such media have a finite capacity and stop working once they are saturated.
Measuring silicate
Most test kits are based on the molybdate reaction. In acid solution, silicic acid and silicate react with molybdate to form a yellow silicomolybdate complex whose colour is proportional to silica. EPA Method 370.1 covers 2–25 mg/L with the yellow complex; for concentrations below 1 mg/L, the complex is reduced to a more intense heteropoly blue. The method measures only molybdate-reactive (dissolved) silica, not colloidal forms; laboratories use ICP spectrometry for total silicon, with reporting limits of about 0.05–0.5 mg/L.