They are numerous in salt water tanks because of the tank water's high calcium and silica content, the hardness and the high alkalinity. This species usually looks like a carpet of rust-colored algae. Their exterior cases consist mostly of silica which they extract from the water in enormous quantities. They are, in a sense, micro computer chips, and are so intricately designed that they are used to test the optics in high end optical instruments. Diatoms convert the products of photosynthesis to oils and fats, unlike plants which convert them to carbohydrates.
Diatoms are perfectly symmetrical.
There are approximately 10,000 species of Diatoms, and the fossil record shows that some lived more than 66 million years ago.