More common in newly started tanks, Brown algae is quite a nuisance and can be present in a tank for it’s entire lifespan. It primarily grows on the glass and is easily wiped off, but can also settle on ornaments or the leaves of plants and starve them of light.
The three main causes of these bacteria growing are food, light and lack of water flow:
The food source most commonly found for diatoms are the fish food your fish fail to eat. If this food remains in your tank it not only pollutes the water, it becomes food for other organisms. If you see Brown algae you are, unfortunately, likely to be overfeeding your fish.
Light is required for most life to exist and for brown algae it is essential. Your aquarium lights should not be turned on for more than 10-12 hours each day. Most people won’t want to adjust what lights they use just because of algae, but your choices are to either buy less powerful lights or to have them turned on less.
The lack of water flow can be something many people will not consider. When I first saw this in my tank it was because the filtration units I had were full of gunk and this gunk had reduced water flow in my tank. The filtration of the food waste was also impacted because of the gunk so the problem grew exponentially. Diatoms find it difficult to grow where the water flows quickly, so putting in a new filter insert (or rinsing it in tank water) can really help speed up the flow and reduce diatoms.
So basically, try feeding your fish less, try reducing lighting and try increasing water flow. If that fails, or you aren’t willing to do them, get used to scraping diatoms off the glass. If you do have to scrape, do your water changes afterwards so you can suck most of the loose debris out as you do it.
It should be noted that these are more common in new aquariums because the silicone used to seal aquariums can be a source of additional food. They can, however, grow at any time in any tank.