Brown Algae (Diatoms) In A Fish Tank – Causes & Removal

    Brown algae (diatoms) in a fish tank

    You might not realize you have brown algae in a fish tank at first. Rusty brown and spotty, it does not stand out as readily as green algae does. But given time, it can cover your entire tank in a coating of brown dust. What is brown algae, anyway?

    Brown algae is caused by an overgrowth of diatoms. These organisms thrive when light, silicate, and nutrient levels are too high. Brown algae is easy to wipe away but more difficult to eradicate.

    If you’ve never seen it before, you might be confused on how to proceed. Fortunately, I’ll be going into detail on how to both scrub and eliminate it right here!

    What Is Brown Algae In A Fish Tank?

    Technically speaking, brown algae is not a true algae. At least, it’s not the same class of organism as green algae or red algae. Brown algae are better known as brown diatoms.

    And diatoms are some of the most important organisms on earth. These single-celled algae are photosynthetic, meaning they create food from light and carbon dioxide.

    Diatoms are found in huge numbers in freshwater and marine systems all over the world. They are estimated to produce anywhere from 20% to 50% of the oxygen in the air we breathe.

    Diatoms also stand apart from green algae because brown algae loves silica. It’s also called silica algae since the organisms use silicate (SiO2) in its cell walls. Instead of cellulose like true algae and plants do.

    If you try to identify brown algae species, you will need a microscope. When you do take a look, you’re likely to be amazed! Diatoms look like an ugly brown dust from the scale of our eyes. Yet they are incredibly beautiful when seen on an individual level.

    Their silica (glass) skeletons reflect a rainbow of colors. Each diatom species is as unique as a snowflake; some are angular and boxy, others are round. Many more have spikes, spokes, corners, and other improbable features.

    They are all works of natural art. In fact, diatom art was very much in vogue in Victorian England. Artists would arrange the diatoms into patterns to be viewed under a microscope. Nowadays we prefer selling diatom art as photographic prints!

    Diatoms from freshwater and marine tanks are of different collections species. But both have a similar origin.

    Interested in learning more about brown algae? This video does an excellent job at explaining what diatoms are. As well as their importance to the wider world:

    YouTube player

    Is Brown Algae The Same As Green Or White Algae?

    Green algae is a very different organism. These are true algae and grow attached to hard surfaces. Or suspended in the water column as green aquarium water. They share similar causes but are controlled in different ways.

    There are many species of freshwater fish and invertebrates that are willing to eat green algae. It is an important source of food for molly fish, goldfish, plecostomus, snails, and other animals.

    But few animals eat brown algae. The silica skeletons make diatoms mostly indigestible.

    White algae is not a true kind of algae either. It is caused by colonies of fungi and bacteria that flourish when nutrient levels get out of control. Decaying fish bodies, fish food, new driftwood, and dead plant leaves are what white algae loves to consume.

    Since it’s not photosynthetic, lowering light levels won’t do anything to it. White algae can also be infectious to fish if water parameters become too toxic from free floating fungal spores.

    Where Does Brown Algae In A Fish Tank Grow?

    It is easy to identify brown algae diatoms. They start out as a faint rust-colored dust that starts to collect on hard surfaces.

    The tubing of your filter intake, the glass panels of fish tanks, rocks, driftwood, and plant leaves start to turn brown. Even your aquarium gravel can take on a coating.

    Brown algae is photosynthetic so it does need light to grow. It will grow most vigorously where light levels are high. Decorations directly under your fish tank lights will show the most brown.

    But even the shadowy areas will start to grow brown algae as these organisms don’t need much light to survive. In nature you can find brown diatoms at depths far beyond where plants and even green algae can survive.

    How do brown diatoms get into your fish tank? They are everywhere in the world. Diatoms hitchhike into your clean tank with new fish or plants. They even floating on air currents or inside dust motes as spores.

    Diatom spores are incredibly hardy and long-lived. One study found that diatoms can survive in spore form for thousands of years.

    What Causes Brown Algae Diatoms To Grow In A Fish Tank?

    What causes brown algae to grow in a fish tank
    Bushymouth catfish (Ancistrus dolichopterus)

    Now we better understand what brown algae is. It is found everywhere in nature, even in healthy fish tanks. So how does it start to grow out of control?

    Too Much Light

    Brown algae is a photosynthetic organism so it needs light to grow. Lights that are too intense or run for too long will cause it to spread faster.

    With most aquatic life, you need to have photosynthetically active radiation (PAR) for good growth. Lack of PAR is the main reason why so many beginning planted tank keepers fail.

    They try to use basic fish tank lights on live plants. Which tend to be too warm or too cool to fuel photosynthesis.

    Brown algae is much better at using subpar lighting than true plants are. It will grow even under the low-power incandescent lights found in cheap aquarium setups.

    That said, reducing light levels to control brown algae does not work very well. Brown algae needs very little light to survive; much less than green algae or green plants. Any live plants will die long before you kill the brown algae on aquarium surfaces.

    You’d have to keep the lights off for weeks. And even if you do, dormant algae spores will return if the water chemistry still favors algae growth.

    Controlling light levels will help. But stopping a brown algae bloom for good requires more than one strategy!

    Excess Silicates In Your Aquarium Water

    Its debatable whether silica based sand mixes are actually a source of brown algae outbreaks. It is true that brown algae needs dissolved silica to grow. But there are two problems with this often-repeated theory.

    The first is that silica has a very low solubility in water. Sand doesn’t just dissolve; if it did, sand on beaches would be non-existent. Trace amounts of silica might be in your tap water. But it’s also pretty much everywhere.

    Silica is also an important ingredient of any aquarium gravel that contains quartz crystals. Since most, if not all, kinds of gravel have some quartz, you’re always going to have silicates in your substrate.

    And the second reason is that even if you don’t use silica sand, your aquarium glass is also made of silica. Since your fish tank setup is in no danger of dissolving, I don’t think silicates are as big an issue as they are made out to be.

    That said, if you have a silicate test kit, it is a good idea to test your aquarium water and tap water. If levels do test as very high then you may want to switch to a silica-free water source.

    New Tank Syndrome

    New tank syndrome is one of the worst things that can happen when setting up a new aquarium. New tanks don’t have the established suite of good bacteria that mature, fully cycled aquariums do.

    Once ammonia is released as fish waste it tends to accumulate. Since it’s not being transformed into nitrite, the less toxic form, fish die very fast.

    Brown algae actually help with new tank syndrome some by using this ammonia as fertilizer.

    But there is such a rapid bloom of nutrients that brown powder will grow fast all over the glass or decorations in your new tank. Brown algae is a symptom of the much bigger ammonia issue.

    Excess Nutrients In The Tank

    Light is the first input that brown algae needs and nutrients are the second. Nitrogenous waste, phosphorus, magnesium, organic matter…All of these are in abundance in most aquariums.

    And the primary source of any excess nutrients are your fish. Fish waste, uneaten food, and dead bodies are decayed by aquatic bacteria. Which release nutrient plumes into the water for brown algae to use.

    Regular aquarium maintenance includes water changes and changing your filter media. If you don’t have a solid schedule then your tank experiences fluctuations in parameters.

    These are stressful to both fish and plants, which prefer a stable environment. Algae, on the other hand, loves sudden shifts in parameters. Higher animals and plants can’t adjust their biochemistry fast enough to take advantage of the new situation.

    Algae blooms are the end result of letting excess nutrients collect. As well as not doing maintenance on a regular schedule.

    Does Brown Algae Harm Fish?

    Guppy (Poecilia reticulata)

    Brown algae in fish tanks won’t harm the inhabitants. Some infections like velvet and ich look superficially similar to brown diatoms. But fish can’t be infected by diatoms. They are much more closely related to algae and plants than any sort of parasite.

    All brown algae wants to do is photosynthesize and spread in peace. In fact, it’s more beneficial to your tank water than not. Brown algae consume nitrogenous waste products and phosphorus.

    They also suck up the carbon dioxide that fish exhale. Exchanging it for oxygen for fish to breathe instead. Brown algae don’t have the impact that a huge number of plants will. But they are better than no plants at all.

    People don’t like brown algae in fish tanks because it isn’t attractive. It looks fine in natural lakes and rivers. But it gives your aquarium a dirty, unkempt appearance. It even grows over your viewing glass, mudding the interior view of your tank.

    Is Brown Algae Harmful To Aquarium Plants?

    In low amounts brown algae won’t harm your live plants. In fact, most fish tanks have some brown algae in them at all times. The diatoms are simply in balance with all of the other organisms in the aquatic environment.

    But if allowed to grow unchecked through a sudden parameter shift, you can easily have a brown menace on your hands.

    Enough brown algae growth can block plant leaves from getting the sun that they need. Killing them slowly as the coating gets thicker.

    Most healthy plants have defenses to prevent algae from growing on their leaves. Algae of any kind on leaves is a sign that your plants may also be struggling.

    An essential plant nutrient may be exhausted in your tank. Or they aren’t getting enough carbon dioxide to fuel their growth.

    This is a bit of a paradox since I’d said earlier that algae blooms can be caused by high nutrient levels. Well, it’s also true that plants need higher nutrient levels just to survive. Brown algae needs very little compared to plants.

    If your plants aren’t getting phosphorus, carbon dioxide, or another element to grow, their growth halts. Allowing algae to take off.

    Nutrient levels are a fine balance that takes weeks to achieve in a planted aquarium. Which is why stability is ever so important to maintain once you achieve it!

    How To Get Rid Of Brown Algae (Diatoms) In A Fish Tank

    How to get rid of brown algae in a fish tank
    Red swordtail fish (Xiphophorus maculatus)

    Brown algae is not dangerous to fish tanks. It’s just unsightly and indicates a deeper issue with water quality. So how can we get rid of brown algae forever?

    Wipe Away Brown Algae Diatoms

    One easy way to get rid of brown algae is to simply wipe it away. Most algae attaches stubbornly to surfaces. And takes a lot of elbow grease to scrub clean. But brown algae is softer than green algae and very loosely attached.

    It forms more of a film or fine powder over surfaces. And it can be removed by wiping with a soft cloth or algae scrubber for fish tanks. Be sure to choose one rated for your aquarium type (glass vs plastic/acrylic).

    Using a glass algae scrubber can scratch soft plastic panels. Leaving permanent grooves that brown algae can then grow within.

    A soft sponge is better for cleaning the leaves of delicate aquarium plants. Wipe gently to avoid bruising or tearing that can cause infections.

    Once you remove brown algae from the aquarium’s hard surfaces, you then need to do a water change or two. The algae is still there, suspended in the water column. And it doesn’t die from by being wiped from tank surfaces. If left in the aquarium, all that brown algae will return.

    Remember that even if you do physical remove brown algae you still need to address the root causes of lighting and nutrients. Otherwise it will come back in a few weeks.

    Removing Brown Algae From The Substrate

    Removing brown algae that is growing on your aquarium gravel or sand is much more difficult to do. It is soft and easy to wipe away but it’s not feasible to wipe every gravel grain.

    Start by using a gravel vacuum. Vigorously plunging the gravel vacuum in and out of the gravel will dislodge much of the brown algae. This may be enough for a light bloom.

    If the brown algae diatoms still cling stubbornly, you may want to drain the tank and remove the substrate. Place it into a bucket and churn it by hand or with a large hand tool.

    You can also run a water hose into the bucket on full blast. This method is time-consuming but will eventually remove all traces of brown algae from your gravel.

    Add Diatomaceous Earth Filter Media

    Believe it or not, you can use diatoms to control brown algae diatoms! Diatomaceous earth powder is made from the fossilized skeletons of marine diatoms. They lived millions of years ago and died, falling to the sea floor.

    These diatoms eventually become rocks. Which are then ground up to release the diatom skeletons for human use.

    One excellent use for diatomaceous earth is as a water polishing agent. The skeletons are so tiny that they can screen out microscopic particles when water is pushed through it. Including other, living, diatom skeletons.

    All you need to do is add food grade diatomaceous earth to a customizable filter media bag. The bag should be sized according to the type of filter you have (canister vs power filter). As well as the size of the media chamber.

    But you need to change it weekly since it clogs very fast. Fortunately, it is very cheap to purchase and quick to replace.Diatomaceous earth is very effective at pulling out the diatoms that get suspended in the water after you wipe away brown algae by hand. It will also pull out green water algae.

    Canister filters are much better at using custom media cartridges than power filters, however. Their design forces most incoming water into fine media like diatom powder. Whereas power filters sometimes allow water to bypass areas where flow is restricted.

    Reduce Nitrogenous Waste Levels

    One major part of regular tank maintenance is checking nitrogenous waste levels. Ammonia is the one that most aquarists are worried about. After all, it is the most toxic fish waste product. But nitrite and nitrate also need monitoring.

    Brown algae typically grows in aquariums where nitrate levels are high. Nitrate is the least poisonous waste product. Water parameters of 10-20 parts per million are well tolerated by freshwater fish. But it is also a good source of fertilizer for all kinds of algae, including brown algae.

    Nitrate is the end result of the aquarium nitrogen cycle. There are de-nitrifying beneficial bacteria that eat it, converting it into nitrogen (N2) gas. But they only live in anaerobic environments, which are rare in a healthy fish tank.

    So nitrate has to either be removed by you when you do water changes. Or by plants, algae, corals, and other photosynthetic aquatic organisms.

    If you don’t have any live plants or corals to feed, then try and get nitrate levels as low as possible. The better your water quality the less brown algae you will see.

    Control Phosphate Levels

    Much has been written about nitrogenous wastes. But plants, algae, and diatoms also need phosphorus for growth. Phosphate (PO4) comes from fish waste and leftover fish food.

    Low quality brands tend to have much more phosphorus since it’s not widely known to be a pollutant.

    Fish only need trace amounts of phosphorus as a nutrient. The rest gets excreted, where algae use it instead.

    Take a moment to read the ingredients label on any fish food brand you go with. The lower the phosphate levels the less it will contribute to brown algae diatom growth.

    Use Reverse Osmosis Water

    The best way to both control and prevent brown algae diatoms is to use reverse osmosis (R.O.) water. R.O. water is run through an ultra purification process using fine membranes and high pressure.

    This screens out dissolved minerals and other molecules contained in ordinary tap water. Leaving you with water that has little in it except small amounts of carbon dioxide, oxygen, and other gases.

    R.O. water is also free of the silicates that silica algae need to build their cell walls. By using it, algae growth slows and then stops entirely.

    There are a few downsides to using reverse osmosis water in your aquarium. While it is very pure, the lack of minerals means that pH and other parameters can change very quickly.

    Dissolved minerals help buffer water towards alkalinity or acidity. Pure water can leach huge amounts of minerals from your substrate or rocks Causing pH swings that are fatal to your pets if not accounted for.

    If you don’t have your own reverse osmosis generator, buying R.O. water for every water change can be expensive as well.

    Tap water is much more convenient for aquarium maintenance. But switching to R.O. water is one of the best ways to get rid of brown algae blooms.

    Add More Live Plants

    Live plants do a lot to control brown algae diatoms. They are direct competitors for the same essential nutrients that algae need. Nitrogenous waste is fertilizer for them too.

    Plants also suck up carbon dioxide that algae would use for growth. They collect the same light that algae needs. And enough plant leaves near the surface will create shade. Making it that much harder for brown algae blooms to occur.

    Floating plants are my favorite recommendation if you want all-natural ways to get rid of brown algae. They do all of the above and tend to reproduce very fast.

    Floating plants also need little in the way of strong lighting since they live right near the bulb. They also don’t need carbon dioxide enrichment because they get enough from the air.

    Increase Water Movement

    Last, adding some extra current will help prevent brown diatoms from taking hold. This type of algae does not attach as securely as green or red algae do. Aquariums with currents tend to sweep away brown algae patches, preventing them from spreading.

    If your filter has an adjustable outflow, try directing it so currents hit spots where brown algae likes to grow. You can also add a powerhead or circulation pump to the tank. Direct it onto tank walls, if possible.

    Water movement has added benefits besides brown algae control. It aerates the water, increasing oxygen levels and reducing carbon dioxide content.

    Many fish also enjoy the exercise of fighting a current. Especially stream dwellers like giant danios, barbs, rainbowfish, tetras, plecostomus, and Denison barbs.

    How Long Does It Take To Remove Brown Algae (Diatoms) In A Fish Tank?

    Betta fish (Betta splendens)

    Once you’ve tackled the root issues of a brown algae bloom, it will fade in a couple of weeks. That’s how long it takes for the brown diatoms to die and detach from aquarium surfaces.

    I also recommend physically removing brown algae at the same time, however. Diatoms are very hardy but scrubbing them away does disrupt their growth. They may reattach or be trapped by your filter, if you have filter media fine enough.

    Getting organic nutrients, light, and nitrogenous waste levels under control are what’s most important, however.

    Freshwater Aquarium Brown Algae Eaters

    Most fish tank keepers have algae eaters to help keep the green stuff at bay. They come in a wide range of forms.

    Including large algae eating fish like pleco catfish (Hypostomus plecostomus). Small algae eaters like dwarf otocinclus (Otocinclus vitattus). And invertebrates like snails and freshwater shrimp.

    Unfortunately, nearly all aquarium algae eaters ignore brown algae. Brown algae, being diatoms, have indigestible silica skeletons. Animals can digest cellulose found in plants and true algae. But silicate spicules are too strong a defense.

    You can try algae specialists like nerite snails. They are known to eat brown algae sometimes. But they will focus on any green algae first before trying the less appetizing brown kind.

    • Common Names: Zebra Nerite Snails
    • Scientific Name: Neritina
    • Origin: Worldwide
    • Length: 1 to 2 inches
    • Ease of Care: Very easy


    Aquarium brown algae is something most fish keepers will encounter during their hobby lifetime. It’s a little more difficult to handle than green algae since nothing wants to eat brown algae. But armed with the knowledge in this brown algae control guide, you’ll have your tank rust-free in no time!

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