Brown algae (Diatoms)

In order to get answers to basic questions all around algae in the aquarium, we recommend you read this article first.

Diatoms form brown algae layers that classically appear in a newly set-up tank after about one to three weeks. These algae coat every surface in the tank, like the substrate, technical equipment, plant leaves and decoration. They can form very stubborn brown coats on the glass of low-light tanks. Diatoms are also called Bacillariophyta. The class contains many genera and species. Aquarium keepers often call them "brown algae" due to their color. This term is not quite correct though, as true brown algae (Phaeophyceae) are another group, which, with a few exceptions, does not occur in freshwater....

Common causes

The appearance of diatoms during the cycling phase of a tank is absolutely normal, and you need not take any countermeasures. As a rule, the diatoms are crowded out by green algae a few weeks later in the life of your new tank, and they will not reappear.
Diatoms need silicate to grow, more exactly, silicon dioxide, since they construct their box-like cell walls from this substance. If you use tap water with high levels of silicate in your tank, diatoms have sufficient material to reproduce in a new tank. A lack of competitors like other algae and microorganisms will boost their reproduction rate.