Silica (SiO2), Silicic Acid (H2SiO3), Silicate (SiO32-) and several other Silicon (Si)- compounds, are normal constituents of seawater and freshwater. These substances usually occur in low concentrations and are non-toxic.
Silicon compounds would be of little relevance to the aquarist, if it were not for the fact that certain microscopic algae called diatoms, use Silica to build their protective covering, that completely encloses the individual cells.
Diatoms are always present in small amounts in any aquarium, but if they reproduce to greater densities, their presence can be detected without a microscope as a dark brown growth on all objects in the aquarium.

This phenomenon nearly always occurs in a newly set up marine tank, where the clean sand, rocks etc. turn dark brown for several days to two weeks. This brown growth consists of millions of diatoms, that form a pioneer vegetation on the formerly barren surfaces. As the dissolved Silica is used up, the diatoms gradually decrease their numbers to make space for other settling organisms.
This initial diatom-bloom, which disappears after 1-2 weeks, should be regarded as a natural and beneficial phase in the development of a new aquarium.