Brown Algae (Diatoms) In Fish Tank – Causes And Treatment

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Brown algae is a type of diatom that often grows in fish tanks, creating a dirty brown coating all over your decorations, viewing panes, substrate, and even your plants.

Brown diatoms are one of around 16,000 different species of diatoms. And did you know that some are used as a base for dynamite?!

So, what causes this unsightly problem in your tank? Can brown algae harm your fish? And what creatures are brown algae eaters?

Read this guide to find out how to get rid of brown algae if it ever invades your aquarium!

What Is Brown Algae And What Does It Look Like?

Brown algae, fucus

If you’ve tried all the usual recommended methods of algae removal on the brown menace, you’ll know that they don’t work. 

So, why is that?

Well, brown algae is actually not a species of algae, so regular algae treatment isn’t effective. Brown algae are colonies of tiny, microscopic animals called diatoms. 

Brown algae is also called Silica algae. But why?

What’s Silica?

Macro of silica gel on blue background

Silica is formed when Silicon combines with oxygen. Silicon is the basic compound that you find in quartz, sandstone, and glass. In the aquatic environment, Silica is plentiful, and the diatoms use it to create a protective outer “clamshell” around their cell wall. 

Brown Silica algae are found in both freshwater and saltwater home fish tanks, using light to photosynthesize and forming soft, brown patches that coat all your aquarium surfaces, including the glass viewing panes, substrate, plants, and decorations.

Is That Really Brown Algae?

To confuse the issue, there are true brown algae in the Class Phaeophyceae.

These algae are true algae rather than diatoms and include many cold marine plants, such as kelp. There is also Golden Algae (Class Chrysophyceae), a large family of brownish-yellow colored algae that occur mostly in freshwater habitats. These algae are single-celled organisms, some of which are flagellated with a tail that propels them through the water. Both these kinds of brown algae are not found in home fish tanks.

Does Brown Algae Mean Your Tank Is Cycled?

Brown algae typically appear in immature tanks that are still being cycled, but fortunately, the problem usually resolves within a few months once the aquarium is established. 

However, diatoms are part of your tank’s natural ecosystem, so you can expect to see the colonies making an appearance from time to time.

Is Brown Algae Harmful To Your Fish?

Generally, brown algae won’t harm your fish and other livestock, provided you keep it under control.

Some fish species eat diatoms, helping to clean the aquarium surfaces. That said, brown algae are not desirable in your tank simply because it’s so visually unappealing.

Interestingly, brown algae can help your fish to thrive! The diatoms consume CO2 and release oxygen, increasing the levels of dissolved oxygen in the water, which is great news for your fish.

What About Plants and Corals?

Unfortunately, brown algae can be fatal for plants and corals. 

The coating of brown algae won’t kill the plants, but it will block out light and starve the plants of nutrients.

What Causes Brown Algae?

There are several causes of brown algae in aquariums:


The primary cause of brown algae blooms is the presence of silicates in the water. Silicates are used as a food source by diatoms, and they’ll thrive in a tank that’s high in silicates.

So, where does silicate come from?

Silicate gets into your tank from several sources, including:

  • Tap water
  • Well water
  • Salt
  • Live sand and live rock

Some silica-based sand mixes can also leech silicates into the water.


If no silicates are present in the water, brown algae can survive on nitrates.

Nitrates are often present in aquarium water. Tap and well water both contain nitrates and if you add plant fertilizer to your tank, that often contains nitrates, too. Of course, decomposing organic matter such as fish food, fish waste, and plant debris also produces nitrates.

If your aquarium filtration system is working properly and the tank is fully cycled, there should be a maximum of 20ppm nitrates in the water. However, in a new tank or one where the water quality is poor, there will be more nitrates for the diatoms to feed on.


Phosphates are also commonly associated with a brown algae bloom. Phosphates generally come from waste products that are decaying in your fish tank, including:

  • Fish food
  • Rotting plant matter
  • Fish waste
  • Dead algae
  • pH and KH buffers
  • Tap water
  • Well water

Regardless of whether your tank is freshwater, saltwater, or brackish, brown algae will thrive if the environment is rich in the above nutrients.

Low Lighting

Some hobbyists blame poor lighting for brown algae blooms, although that’s not certain. As long as the necessary nutrients and silicates are available in the water, brown algae don’t seem to care whether the tank is brightly lit or not. 

How To Get Rid Of Brown Algae

Although brown algae do look disgusting, getting rid of an outbreak is relatively easy to achieve.

Cleaning The Viewing Panes

Regardless of what material your fish tank is made of, brown algae can simply be wiped away from the viewing panes.

Wipe the glass carefully, working from top to bottom, using a smooth sweeping motion. That prevents the diatoms from floating away in the water to colonize another area of the tank.

I find that a small non-abrasive pad works very well. However, you can use a cloth or a sponge, too.

Cleaning The Substrate

Cleaning the soil in the aquarium with a siphon

You’ll need to use slightly different methods to clean your substrate, depending on what medium you’ve used.

Gravel Or Pebbles

If you have gravel or pebbles as your substrate, the easiest way to get rid of brown algae diatoms is to use an aquarium siphon vacuum.

Focus on vacuuming only the top layer and surface of the substrate to suck up the diatoms. Try to avoid vacuuming too deeply into the substrate, or you risk removing the beneficial bacteria that you need for biological filtration, which could make the problem worse.

If the layer of brown algae is very thick, you might need to remove the surface layer of substrate and wash it separately outside of the tank. The easiest way to do that is to place the substrate in a bucket and wash it under running water, stirring the substrate to free and dislodge the algae.


Sand dunes isolated on white background

Brown algae are pretty easy to remove from sand since the diatoms only sit on the very top layer.

All you need to do is use a fish net to lift away the algae without removing the sand. If you have a vacuum, you can suck up the algae. If the algae are stubborn and won’t shift, use a sharp object to lift it, and then suck the diatoms up with the vacuum.

Coral and Live Rock

Use a turkey baster to blow water over the coral. That should push the diatoms off the coral and into the water, where they will be dealt with by your filter.

Live Aquarium Plants

Use a soft sponge to very gently wipe away brown algae from the plant leaves. Your filtration system will remove any algae that are left floating in the water.

Artificial Plants and Decorations

Asian women set the fish tank

Remove any artificial plants and other decorations from the tank and clean them separately.

Make up a bleach solution, using a ½ cup of bleach per gallon of water. Soak everything in the solution for ten to 20 minutes until the brown algae float to the surface.

Rinse the decorations under running water, and wipe them down with a soft cloth or sponge.

How To Prevent Brown Algae In Your Aquarium

Having removed all that nasty brown gunk from your aquarium, you want to make sure that it doesn’t come right back.

1. Efficient Filtration

Beautiful planted tropical freshwater aquarium with fishes. Aquascape.

To keep your tank healthy and safe for your fish and livestock, you must have an efficient filtration system. 

Your filter should circulate the water around your aquarium at least four times every hour. So, check the GPH (gallons per hour) rating on your filter to make sure it’s suitable for the size of your fish tank.

For your filter system to run efficiently, you must rinse the media through in tank water once a month or so to remove any sludge that would clog the media and prevent the water from circulating properly. Depending on the kind of filter media you use, you also need to replace it from time to time. 

2. Water Movement

close up image of landscape nature style aquarium tank with a variety of aquatic plants inside.

Good water movement prevents brown algae diatoms from forming colonies and fastening themselves to your tank surfaces, ornaments, and decorations.

Provided that it’s suitable for the size of your fish tank, your filter system should ensure that water is circulated adequately. However, you could boost circulation by using a powerhead or wavemaker.

Before you increase the current in your tank, check to make sure that the fish you keep can tolerate the water movement you’re creating. If your fish become stressed, their immune systems may be compromised, causing health problems.

3. Water Changes

You need to carry out regular partial water changes to keep your fish tank healthy. However, as previously mentioned, tap and well water can contain lots of nutrients that brown algae will feed on.

Instead, try using Reverse Osmosis (RO) water that contains no impurities. Rather than buying bottles of RO water, which can work out quite expensive in the long term, you could produce your own using a Reverse Osmosis Filter System.

4. Avoid Overfeeding

Any food that your fish don’t eat will rot in the substrate, providing nutrients for brown algae. So, only offer your fish what they’ll eat in a couple of minutes to avoid waste.

It’s also a good idea to have one day per week when you don’t feed your fish at all. That’s not cruel. Fasting allows any food that’s still in the fish’s digestive tract to be processed and pass through the intestines without adding more food to the load. The strategy helps to prevent health problems such as constipation, bloat, and dropsy.

And you won’t be adding uneaten food to the tank where it will decompose, producing nitrates that the brown algae thrive on.

5. Don’t Add Silicates To Your Tank!

Whenever you add anything new to your aquarium, such as live sand, rocks, and salt mix, double-check that the substance doesn’t contain any silicates.

Brown algae love silicates, and they will thrive in a tank that’s rich in the substance. So, by ensuring that you don’t add more silicates to the tank, you’ll be reducing the diatoms’ nutrient source, effectively starving it, and retarding its growth.

6. Filter Media

Some kinds of filter media can be used to starve the brown algae by removing its food source.

Try using a filter media such as Phosguard that contains aluminum dioxide. Aluminum dioxide absorbs phosphate and silicates, removing them from the water and basically removing the diatoms’ food source.

7. Dosing

If you have a marine or reef tank, you could try dosing the water with NO3:PO4-X, which is a chemical that reduces nitrates and phosphates in saltwater environments.

NO3:PO4-X basically promotes the growth of good bacteria that thrives on the same nutrients as brown algae, effectively outcompeting it for food and starving it out.

8. Lighting

Some aquarists say that increasing the lighting in the tank can help to get rid of brown algae.

However, that tactic can also encourage the growth of other species of algae, so you might need to experiment with how much light you provide in your tank each day. Also, not all fish species appreciate lots of bright light, so you may need to add more decorations or plants to your setup to provide shade.

What Algae Eaters Eat Brown Algae?

Siamese algae-eater in freshwater aquarium. Crossocheilus oblongus.

You can help to control brown algae by introducing fish and other creatures that feed on it. However, that approach could prove problematic if you already have an established community of fish in your aquarium. 

Before buying new fish or other critters to add to your tank, you must check that all parties will get along well together and share the same demands as regards water parameters and food. 

Also, you need to be certain that your new additions will eat the brown algae and won’t ignore it in favor of your fish food. And what happens once the algae have all gone? You definitely don’t want your new fish to starve.

So, basically, you need to do your research before you go out to buy new fish or other critters to introduce to your aquarium as algae eaters.

What Freshwater Fish Eat Brown Algae?

Antcistrus, spotted aquarium catfish close-up. Background for pet shop

If you decide to go down this route, there’s a wide range of freshwater fish and other creatures that will live happily on a diet of brown and other species of algae. However, you may need to supplement their diet with algae wafers or something similar once the supply of algae has dwindled.

Freshwater algae eaters include:

  • Ramshorn snails
  • Amano shrimp
  • Twig catfish
  • Nerite snails
  • Bristlenose pleco
  • Malaysian trumpet snail
  • Siamese algae eater
  • Otocinclus catfish 

How About Saltwater Algae Eaters?

There are also quite a few saltwater species that eat brown algae, including:

  • Trochus snail
  • Blennies
  • Tangs
  • Emerald crab
  • Kole tang

That’s just a small selection, as there are many other fish species that eat brown algae. Have a chat with the staff in your local fish store for more suggestions for what fish would best suit your setup.

In Conclusion

I hope you enjoyed this guide to brown algae and how to remove it from your aquarium. If you did, please remember to share!

Brown algae are an unsightly menace that you don’t want in your fish tank.

Colonies of brown diatoms are relatively easy to remove from the viewing panes and ornaments in your tank, and you can prevent their return by following the tips and tricks we’ve provided for you in this guide.

Did you have brown algae in your aquarium? Tell us how you managed to get rid of it in the comments box below.

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

3 thoughts on “Brown Algae (Diatoms) In Fish Tank – Causes And Treatment”

  1. I had a bad outbreak of brown algae about 2 months after I set up my 55 gallon community tank. I had about 10 fish at the time & the diatom hit! Within 3 days it was everywhere. Not wanting to put any type of chemical or other products to get rid of it I did some research & I decided to go with a school of Ottos. I purchased 6 from my LFS & within a week it was just about gone. Those little guys did a great job.

  2. First of all, Ms. Page, you’re absolutely beautiful! And I love reading your articles. Very informative and educational!!

  3. Have Diatoms in 2 large tanks. Both are not new filters and have been set up for many months.
    I’m using Phosguard and Sili out in both tanks.
    Both tanks are over filtered with good levels of flow. I even get Diatoms in my filter inlets and outlets and on air stones.
    My water is hard.
    I’ve tried everything – low light, high light.
    Currently doing 40 – 60 % water changes every 48 hours to keep on top.
    Heavily planted tanks mean its impossible to wipe off every plant.
    I have a third tank which doesn’t have Diatoms – so it can’t be my water source.
    I use Seachem prime and my water test results show very low levels of Nitrate and zero ammonia.
    Ideas please ?


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