Diatoms are a type of algae, they often manifest in a freshwater aquarium as brown dust, usually on the glass and rocks, sometimes also on plants. This algae is not uncommon in aquariums which do not have plants or have very few plants (as this one currently does) and is usually associated with ammonia in the water. It should disappear with the addition of more plants and time.
Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons, fans, zigzags, or stellate colonies. Diatoms are autotrophs within the food chain. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silica (hydrated silicon dioxide) called a frustule. These frustules show a wide diversity in form, but usually consist of two asymmetrical sides with a split between them, hence the group name. Fossil evidence suggests that they originated during, or before, the early Jurassic Period. Diatom communities are a popular tool for monitoring environmental conditions, past and present, and are commonly used in studies of water quality.
The Bacillariophyceae (diatoms) class was described by Ernst Haeckel in 1878. Haeckel was also responsible for Kunstformen der Natur, a book of lithographic and autotype prints which is now available on Wikimedia Commons.