Brown diatoms, or also known as brown algae, gravel algae, or even silica algae is a common annoyance to new and experienced keepers of all types of setups.
Brown diatoms will in fact look brown in color, and can appear on almost anything that is within the water. In many cases, the first place that it will appear may be the intake our output tubes to your filter, and possibly any decorations that are within the aquarium. Depending on any sunlight factors, it may appear on a glass panel first before anything else. Normally this algae is referred to as just "diatoms", since the algae is just small little atoms that do not grow in size but will rapidly grow number wise.
Whenever cleaning out any filter media, you may notice an excess of brown diatoms. This is due to the fact that they can live under little to no light conditions, and the water that is being pumped through the filter media is mostly full of organic waste and perfect for both the beneficial bacteria and brown diatoms to feast on.
Brown diatoms will normally always be the first type of algae that is found within any water setup, from aquariums, to ponds, to even stagnate water. This is simply due to the fact that the water parameters have yet to fully stabilize (as before nothing was adding in anything to change the parameters such as waste, food, or even light from a light fixture of sunlight). If found within a new setup, wait it out for a few weeks until the water parameters stabilize. Once this happens, the diatoms should kill themselves off as they will be starving to death (or the inhabitants may make a quick snack of this tasty meal).
There are many different theories as to why this algae is the first to populate a new setup. Many state that if the silicate to phosphate ratio is high, than the brown diatoms will have a higher growth rate compared to other forms of algae. However as the ratio balances out (and even if the phosphate has a higher ratio than silicate) the conditions will favor no specific algae, or favor another one which will make the brown diatoms compete with other algae that may be growing.
In a newly established setup, the only prevention methods would be to make sure that you are keeping up with water changes on a frequent schedule. Since this type of algae is easy to clean, taking a cloth to rub off any that is growing will reduce its numbers for the time period. Another cause in newly established setups is that there is too much waste and no beneficial bacteria to eat up the toxins, thus making sure that the nitrogen cycle is fully completed before adding any inhabitants in is highly beneficial overall.
In already established setups, it is suggested to check the water parameters of the water. Normally this algae feeds and grows in conditions that have excess waste, decaying matter that may be trapped in the substrate, or in locations that are getting direct sunlight for a few hours during the day. Since this algae can grow in little to no light conditions, adjusting the light levels will slowly decrease the growth rate but have little to no overall impact.