Silicates (SiO2)

Silicates fuel the growth of diatoms (brown algae). They are common in new aquariums and can enter from unfiltered mains water.

Detailed explanation

Silicates are silicon compounds dissolved in water. Diatoms use them to build their shell (frustule). Where there are silicates there are diatoms: that brown patina that covers everything in the first weeks of an aquarium's life.

The main sources are: mains water (if not filtered with reverse osmosis), new substrates (silica sand, rocks), some cheap sea salts.

In a new aquarium the diatoms explode in the first 4-8 weeks then become exhausted as the silicates in the substrate are consumed. This is normal and does not require intervention.

The problem arises when the silicates continue to arrive: RO water with exhausted membrane, topping up with mains water, substrate that continues to release them.

For marine aquariums silicates should be below 0.5 ppm. For freshwater aquariums they are less critical unless diatoms become a persistent problem.

In ponds, diatoms are less problematic because they are consumed by other organisms in the richer and more diverse system of the pond.

Reference values by aquarium type

How it is measured

Specific drop test (Red Sea, Salifert). ICP analysis. The TDS metre does not distinguish silicates from other dissolved solids.

What happens when it is too low

No problem.

What happens when it is too high

Persistent diatoms that cover glass, substrate and decorations with a brown patina.

Related terms