Brown Algae In Betta Tank

What is this brownish film covering the surfaces of my new tank? What’s wrong? How much should I be concerned?

It is probably diatoms, also known as brown algae. They might look appalling, but they are not harmful, and more often than not they will go away by themselves.

What Is Brown Algae?

Brown algae is a special kind of microscopic algae called diatoms. Diatoms are unicellular, they form colonies and use light to photosynthesize. They are able to utilize nutrients like silicate, phosphorus, and nitrates to produce energy.

Actually, diatoms are ubiquitous in water environments. They produce up to 50 percent of all oxygen produced on the plant every year. They are a substantial food source sitting at the bottom of the food chain.

Diatoms form a fascinating outer cell glass-like structure of silica called frustule.
In an aquarium, brown algae look like a thin brownish film covering all surfaces, like the substrate, stones, glass, and plants.

This is especially true in new freshwater aquariums. Diatoms are one of the first organisms to multiply and take advantage of abundant nutrients.

After some weeks when excess chemical nutrients have been utilized and when plants start competing for nutrients, brown algae will go away on their own. Diatoms are often part of what is called new tank syndrome.

If you get brown algae in an established tank, then some aspect of your tank environment needs to be addressed.

Is there too much silica in your tank?

Silica from the substrate

The source of Silicon dioxide (SiO2), also known as quartz or silica, may be your substrate. This is especially true if you have recently added new sand. All kinds of sand contain silica, whether they are sold for aquarium tanks, pool filters, or play sandboxes. Rinse it well before using it to wash away sand dust. Pair sand with a good filter or opt for other kinds of suitable substrate.

Too much silica in your tap water

Your tap water may contain high levels of silica. Test your water with a test kit. Test kits have a reputation for being unreliable, but they get the job done. If you want more accuracy, take a sample to a lab and do a proper water analysis. This will be costly and can take weeks to get an answer.

If your tap water is high in silicate, use reverse osmosis (RO) water or other water sources.

High Nitrate levels

Diatoms feed on nitrates. Nitrates form naturally at the end of the nitrogen cycle. We need to keep nitrates at low concentrations by doing regular partial water changes.

Nitrates are not harmful to the fish if kept low. For Betta fish, optimum levels are up to 10 ppm. You can read more about cycling here.

High Phosphates

Diatoms also feed on phosphates. Phosphate (PO4) builds up from the decomposition of organic matter like food, fish poop, and decaying plants.

Do not overfeed your fish.

You can clean the substrate from organic waste by vacuuming the substrate before performing the regular partial water changes.

Another source of phosphates may be the water you use. As with silicate, test and invest.

So how do you remove brown algae?

Please do not be in a hurry. As discussed above, the problem will be solved by itself more often than not.

Scaping the glass and vacuuming the substrate will help. However, when scraping, some diatoms will suspend in the water and can potentially clog your filter. Do a water change after scraping and regularly check your filter.

Find the root cause

Overfeeding and or overstocking your tank may be the reason for high phosphates and nitrates. Try lowering food portions and try transferring some fish to another tank.

Your aquarium should be well-oxygenated and have adequate lighting. Plants need lights and will outcompete brown algae for nutrients.

Use chemical filtration if needed to lower silica and phosphates.

Diatoms prefer still water. Create some flow to discourage diatoms from attaching themselves to surfaces. Beware though, that bettas do not like strong currents either.

Algae eaters

A lovely natural way to keep diatoms under control is to introduce an algae eater. The best algae eater are snails. However, beware that some snails reproduce fast and can become a pest.

Nerite snails are a good choice since they only reproduce in saltwater. Bettas will tolerate them and mostly leave them in peace.

There are also popular fish algae eaters like Otocinclus Catfish, but you need at least three of them to form a small school. You risk that your betta will pick on these peaceful fish.

UV filter

You can use a UV filter to sterilize water. Ultraviolet (UV) light kills microorganisms like algae, diatoms and microbes. There is a UV bulb in an enclosure that will sterilize passing water. Free floating diatoms will be eliminated before setting on a surface.
Let it run a few hours every day and you will prevent diatom bloom.

Aquarium fertilizers

Are you using fertilizers? Trying cutting down on water additives and conditioners. They may provide nutrients to brown algae and mess up your water parameters.


Brown algae can be annoying. However, they will go away with some good tank management and patience.

About the author

Hi, I am Alex.
My passion is aquariums. I am especially drawn to betta fish. I love experimenting, learning, researching, and writing about them.

You can find the articles I wrote here

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