Brown patches on the gravel or glass of the tank is a kind of film known as a Silica Algae or Brown Algae. Once established, it can rapidly coat most surfaces of the aquarium's interior with a thin, dark brown coating.
This problem is especially common in new aquariums, which may cause concern for people new to fish keeping. The good news is that this issue is pretty easy to clean up; for the most part, it is easily removed. It is also relatively easy to stop brown algae from even growing in your aquarium if you know the cause. A few preventative measures will have your tank looking great and algae-free.
Many Brown-Colored Algaes
The true Brown Algaes, comprised of the Class Phaeophyceae, are a large group of multi-cellular algaes, including many of the seaweeds in cold marine waters, such as kelp. These are not the types of algae that are growing in your aquarium!
Also, the Golden Algae, Class Chrysophyceae, are a large group of yellow-brown algae found mostly in freshwater environments; they are an important food source in food web dynamics of freshwater ecosystems. They are single-celled organisms that have a cellulose cell wall just like a plant. Many are also flagellated—having a tail for propulsion—so they are normally found suspended in the water column. These Golden Algae are also not often found in the home aquarium.
However, there is another kind of algae that grows in aquatic environments; it appears brown, especially under artificial lighting in the aquarium setting. This is referred to in the aquarium trade as Brown Algae or Silica Algae.
What Is Silica?
In its natural state, the basic and abundant element Silicon combines with oxygen; the result is Silica, which is the basic compound in sandstone, quartz, and glass. Since this substance is readily available in the aquatic environment, myriad single-celled organisms use it to create a protective outer "clamshell." The organisms that do this are known as diatoms.
Brown Silica Algaes (Diatoms)
"Brown Algae" is the common name that refers to the diatoms, Class Bacillariophyceae, that find their way into both freshwater and saltwater home aquariums. Diatoms are unicellular organisms that can occur either as solitary cells or in colonies.
Like plants and other algaes, diatoms photosynthesize light into energy. Each species creates a unique opal-like crystalline covering around its cell wall; these appear almost like snowflakes and are quite beautiful under a microscope even though they function as substantial armored protection for the tiny organisms.
Planktonic forms in open water usually rely on turbulent churning of the upper water to keep them suspended. But most diatoms are non-motile, as their relatively heavy cell wall causes them to readily sink to the bottom, creating a thin film across the bottom of any aquatic environment.
Is Brown Algae Dangerous?
In general, Brown Algae diatoms will not harm your fish if you keep them under control. Some fish do like to eat these diatoms and can help to clean up your tank, but Brown Algae is generally not good for the home aquarium environment.
Unlike blue-green algae that can come off in large slimy sheets, these diatoms don't stick together. Patches of diatoms will generally make your tank look less appealing.
Causes of Brown Algae
Brown Algae is a common occurrence in a newly set up aquarium. Aquariums kept in dark places are also more likely to develop Brown Algae problems because the plants and green algaes that grow in bright light compete for the nutrients diatoms need.
Brown Algae is also a sign that the water chemistry of your aquarium is not in optimal balance. After providing proper lighting, improving water quality should be your next concern. In general, you can look at a few main causes: excess silica or nitrate in the water or an abundance of nutrients.
Silica can build up in the aquarium from tap water that is high in silicic acid. It can also leach from some types of substrates that you may be using, such as silica sand. If the problem is due to high silica in the water, and Brown Algae seems to persist, get a special silicate-absorbing resin for the tank's filter.
In addition to possibly being high in your tap water, the nitrates that feed diatoms can build up from uneaten food, dead material, or overstocking fish. Increasing water changes to remove nitrates with slow the growth of these diatoms.
Removing Brown Algae
As the diatoms are merely resting on the bottom, a Brown Algae issue is easy to clean up; no scrubbing is necessary. This type of algae does not adhere strongly to the tank surfaces and is easily wiped away. Just wipe off any tank decorations that may be affected, wipe off all surfaces inside of the aquarium, and then vacuum the gravel.
The hardest part of removal is usually getting it out of the gravel, but vacuuming the gravel with a siphon will quickly remove it. Vacuuming is important to ensure the algae will not grow back as quickly.
In a freshwater aquarium, you can add a sucker-mouth fish that will readily eat brown algae. Stock one plecostomus or several otocinclus catfish to do this job. In a saltwater aquarium, many species of fish and invertebrates are diatom grazers.
Preventing Brown Algae
Increasing the lighting so the tank gets at least eight hours of light per day. As a new tank matures, Brown Algae are often eliminated naturally by plants and green algae competing for nutrients in the water, such as nitrite and nitrate.
As with any algae, keeping the tank clean and performing regular partial water changes are the best preventative measures. The best maintenance measures are summed up simply:
- Add reverse osmosis water to a water source that is high in nitrate or silica
- Change the water regularly
- Clean the aquarium regularly
Unfortunately, it is still possible to get algae in spite of regular maintenance, especially in a newly established aquarium. Prompt attention to sudden algae growth will prevent more serious problems later on.