Algae in the Aquarium

Algae is one of those things that every aquarium keeper encounters sooner or later. Most folks don’t like algae in their tanks, and that’s understandable. It covers the glass, fogs the water, smothers plants, and other terrible things. My own view is that algae can be just as beneficial and attractive as plants in an aquarium, if it is kept under control. Dealing with an algal bloom can be frustrating, even for experienced aquarists.

Plants are relatively slow to adapt to changes in their environment. If your water parameters suddenly shift, your plants may slow in growth while they adjust. Meanwhile algal cells will quickly adapt and explode in number. If algae are allowed to grow densely enough, they can block light from reaching your aquarium plants and prevent efficient photosynthesis. Algae also competes for the same resources as plants, which further weakens them.

One of the dangers algae pose to fish and invertebrates is usually caused by the sudden, widespread death of an algal colony. If a large bloom of algae were to all suddenly die at once (maybe from dosing algaecide), the resulting decomposition could cause a spike in ammonia and CO2. This is not good for fish or invertebrates. Some algae, like blue-green algae, produce toxins that are harmful to fish.

How Does Algae Get Into My Tank?

Algae can come in on plants, used decorations, driftwood, “bag water” from the pet store, and even the digestive tract of newly purchased fish. Some algae spores can even be carried on gusts of wind or in dust on your shoes and clothing.

It’s pretty difficult to 100% avoid any contact with algae. So the question is how to manage it in the aquarium? For starters, it is a good idea to track your aquarium’s water parameters. If you are suddenly experiencing an algal bloom, perhaps there has been some change that has caused it. Maybe the temperature has gone up. Maybe a fish has died behind the plastic castle, and is decomposing. What is your lighting situation? Is your tank receiving too much light or not enough light? Any of these factors in combination can result in algal blooms.

What Kinds Of Algae Should I Be Aware Of?

Below is a list of some common types of algae you may experience and how to treat them. These “types” are simply collective terms for groups of algae that result in common symptoms. There are countless species of algae out there, and it can be impossible to specifically identify which ones are in your tank, so we hobbyists use these generic names to describe symptoms of algal blooms. Each of these types can be made up of many different species working together in a colony.

Green Water Algae, Source: Lisa Brewster

Green Water Algae

Description: Green, soupy-looking water. It can be very dense and cloudy. Green water will begin as a green tint and over time progress into water that can become too thick to see through. Green water algae is made up of a cloud of individual free-floating algae cells. These cells are found in all types of water, and it is only when their numbers explode that they become visible to the naked eye.

Threat Level: Mostly Harmless – It may not look pretty, but isn’t toxic to fish or invertebrates. If a thick bloom of green water was to be killed suddenly, this could cause spikes in CO2 and ammonia.

Cause and Prevention:  The primary cause of green water algae is excessive lighting and nutrients from fish food and waste. You can prevent a bloom by doing away with excess light and food. Avoid overstocking and overfeeding your tank, and keep your tank away from direct sunlight or long stretches of time under bright artificial light.

Treatment: First, do a hefty water change. The water change alone will make it look better immediately, and will remove the majority of the algae and the nutrients they were consuming. But don’t stop there, it’ll only be a couple of days before the algae returns. The best long term solution for treating green water algae is to cut off its abundant light source and food.

If your aquarium is nearby to a brightly lit window, consider using thicker, darker drapes or even moving the aquarium away from the window. I recommend getting a timer for your aquarium lights, and set it to a 545 photoperiod. In a 545 photoperiod, your lights come on at 7AM (for example), remain on for 5 hours, and turn off at noon. They remain off for 4 hours and come on at 4PM. The lights then remain on for another 5 hours, until 9PM. The lights then go off for the duration of the night. It may take a week or two for you to see all the green water disappear. If your green water persists, bring down the total number of hours that the light is on and see if that helps. Continue your regular water change schedule.

If your green water algae still comes back, you can buy a UV sterilizer or use a very fine, micron filter pad in your HOB or canister filter. I’ve actually had luck removing small particles from my water by using batting in my filter media. You just need to make sure it doesn’t have any additives like a fire retardant.

Brown Algae. Source:

Brown Algae

Description: Soft, brown patches. It has a fuzzy appearance, and begins as something that looks like a brown stain on the glass or other surfaces. It clings to surfaces, and can be seen on decorations, gravel, glass, filter pipes, etc…

Threat Level: Delicious And Nutritious! – Brown algae won’t harm your fish or invertebrates. Some species actually enjoy eating it. Snails, plecostomus, otocinclus, and many others will happily graze on it. If it’s been growing for a while, you might see little clumps of it hanging from your filter outlet.

Cause and Prevention: Brown algae is caused by a combination of low light and excess nutrients like nitrates and silicates. It’s often found in newly established tanks. This might be because many kit aquariums come with weak lights that cannot support green varieties of algae. You can avoid brown algae by using brighter lights, doing more frequent water changes, and reducing the amount of food you are putting into the water. If you keep snails or other scum-sucking pets, you probably won’t see much of it in your tank, because they love to eat it.

Treatment: First, you should do your best to scrape the algae from the covered surfaces. For the aquarium glass, there are many handy tools available that will scrape the algae without damaging the glass. Here are some common algae scraping tools that are available online. My favorite is the magnetic floating algae scraper. You can use a dish sponge, but make sure it is only ever used for this purpose. You don’t want to get soap residue in your aquarium.

For algae on decorations, you can put them in a pitcher or small bucket and spray them with a solution of hydrogen peroxide and water. Simply mix ordinary antiseptic hydrogen peroxide (3% concentration) in a spray bottle with tap water. A ratio of 1:4 (peroxide : water) will work fine. Use this with an unused toothbrush to get in there and remove the algae.

While this kind of algae lives on surfaces, it is a good idea to do a large volume water change and vacuuming to remove excess nutrients and their source. Vacuuming will also remove most algae that is sticking to the substrate. Once you’ve scraped, vacuumed, and refilled, consider limiting the amount of food you feed your fish.

Green Dust Algae. Source: David Hall

Green Dust Algae

Description: Green Dust algae looks like a very fine layer of green growing on glass and decorations. It is often noticed blooming on aquarium glass and obstructing the view of the interior of the tank.

Threat Level: Delicious And Nutritious! – It’s kind of ugly and annoying when it grows on the front-facing panel of glass, but there is nothing harmful about this algae. I suppose if it covered the leaves of a slow-growing plant with already too little light, it might cause problems for the plant. Snails and other invertebrates LOVE to eat this algae. If you work your lighting and nutrients just right, you can probably sustain a colony of invertebrates on this algae alone.

Cause and Prevention: Too much light and nutrients will cause this algae to be one of the first you may encounter. Healthy, fast-growing plants and dutiful algae eaters will prevent this algae from ever being a problem. Consider stocking a few shrimp or snails to help keep it to a minimum. If it begins to grow more than just a little here and there, you may want to knock down the total hours per day you have your aquarium light on or move it further away to reduce the brightness. You may also need to examine your water parameters for evidence of too many nutrients from either fertilizers or over feeding.

Treatment: A The first thing you can try is simply waiting it out. Sometimes a bloom of algae like this is triggered by some shift in the aquarium. Maybe you just pruned your plants or moved the light a bit closer. Algae in general are opportunistic and will often surge upon an opportunity while the slower plants are still changing course. If you already have invertebrates like snails or shrimp, they will be more than ready to begin consuming this tasty treat.

If you do not have snails or shrimp that can eat the algae, you should do your best to scrape it from the glass. There are many handy tools available that will scrape the algae without damaging the glass. Here are some common algae scraping tools that are available online. My favorite is the magnetic floating algae scraper. You can use a dish sponge, but make sure it is only ever used for this purpose. You don’t want to get soap residue in your aquarium. As mentioned above in “causes and prevention,” consider reducing light and nutrients.

Green Fuzz Algae. Source:

Green Fuzz Algae

Description: A green algae that looks somewhat like a split between dust algae and hair algae. It can coat rocks, wood, and plants, creating a kind of “fuzzy” texture up to about a 1/4 inch in length.

Threat Level: Delicious And Nutritious! -Invertebrates love to eat this algae. It is not directly harmful to your aquarium, but if it is allowed to grow thickly on plants, it could cause problems for them. Chances are if this algae has exploded in growth, there is already some imbalance in your tank that is weakening your plants.

Cause and Prevention: Fuzz algae often pops up in young aquariums. It is typically triggered by an imbalance in macronutrients. A small amount of this algae growing in a mature tank is not a cause for concern. If you find this algae growing in abundance in a mature, planted tank it could mean that your plants are suffering some kind of deficiency. This is because green fuzz algae should be easily competed by healthy plants.

Treatment: Invertebrates like cherry shrimp and amano shrimp will gladly eat this algae. Keeping a few in your aquarium should keep this algae in check. If it suddenly grows out of control, then you may need to measure your water parameters for any sign of significant changes. If you have been using fertilizers for your plants, you may need to adjust your dosing.

Green Hair Algae

Hair/String/Thread Algae

Description: Long, green, brown, or black filaments. This algae can be seen growing from plants and decorations or in clumps. It can be very annoying if it grows in mosses like Java Moss, because it’s difficult to remove manually. It can be surprisingly strong, and manually removing it can be difficult without accidentally uprooting plants.

Threat Level: Delicious And Nutritious! – Hair algae won’t harm your fish or invertebrates. Some species actually enjoy eating it. Angelfish, mollies, and invertebrates like shrimp or amphipods are some of the creatures that will gladly eat this kind of algae. Amano shrimp are known to delight in eating this algae. If allowed to grow into large, tangled masses this algae could pose a threat to pets like frogs by ensnaring them and preventing them from reaching the surface to breathe.

Cause and Prevention: Excessive lighting and surplus nutrients (especially iron). Brown and black hair algae will usually develop in areas of low light.

As with any algae, the best way to deal with it is to avoid getting it in the first place. Clean any used decorations and rinse thoroughly before putting them in your aquarium. For plants and wood, you can spray them with a weak peroxide solution to kill any algae or spores that might be on the surface. You should research your plants before spraying them with any kind of chemical, because plants may have different reactions based on what species it is. For example, I have found that Amazon Frogbit does not react well to being sprayed with a peroxide solution. If you do treat your plants, always, always rinse them thoroughly afterwards.

Treatment: Honestly, the easiest way to remove this kind of algae is to keep an animal in your tank that will eat it. You can remove much of it manually, but you’ll never be able to do as thorough a job as little shrimp claws. I recommend small invertebrates like shrimp or amphipods. They will quietly keep your tank clean of most types of algae, and they can also be an occasional snack for your fish.

For a quick fix, use a bristled tool like a toothbrush and swirl it around to snag the strands. Be careful not to uproot any plants. Then go out and buy some shrimp!

Beard/Brush Algae on an Amazon Sword. Source:

Beard/Brush Algae

Description: Red or nearly black colored tufts of feathery bristles. Early stages of this kind of algae look like gray blotches. This algae grows on surfaces like wood, rocks, and the leaves of slow-growing plants like anubias.

Threat Level: Harmful to Plants – I accuse this algae of being potentially harmful, because it has the capacity to kill slow-growing plants by blocking light from reaching their leaves. Otherwise it is actually (in my opinion) a visually appealing algae. I would love to have some growing on my rocks and driftwood… but not my plants.

Cause and Prevention: Beard/Brush Algae (often referenced as BBA) thrives in conditions that most algae cannot. It naturally occurs in fast-moving waterways. It does well in tanks with high water flow and low nutrients. You can prevent BBA from taking over by making sure to strike a nutrient balance in your tank that will allow your aquatic plants to thrive. Healthy plants will easily outcompete this algae.

Treatment: BBA can be very difficult to remove manually, and is considered “the worst” in that regard. It’s not very picky about lighting, so increasing or reducing light levels won’t affect it much. Not many species will eat this kind of algae, because it can be quite tough and bristly. Siamese Algae Eaters are reported to eat it, however they tend to be somewhat aggressive and at 6″ long, too large for some aquariums.

Aqua Apprentice has a great video showing how to spot treat BBA using a spray bottle, a syringe, Flourish Excel, and hydrogen peroxide. See the video below:

Is the video not displaying correctly? Click here to be taken to

Green Spot Algae. You can also see evidence of snails feeding on green dust algae from the striped pattern on the glass.

Green/Black Spot Algae

Description: Small bright green or black spots on aquarium glass and other surfaces. Black spot algae will grow in areas with low light.

Threat Level: Mostly Harmless– This algae isn’t harmful, but many don’t like how it makes their tanks look “scummy.” Not many species will eat this kind of algae, so it must not taste very good. Ramshorn snails will eat around it, but Nerite snails will eat it if properly motivated (i.e. if there are no other tasty algaes to eat).

Cause and Prevention: Green spot algae is reported to occur when phosphate levels are low in the aquarium. This can be from going too long without changing water or improperly dosing fertilizers. You can prevent an outbreak of green spot algae by maintaining an appropriate level of phosphate and doing regular water changes.

Treatment: Firstly, use one of the algae scrapers mentioned above to clean the algae from your aquarium glass and then change your water. I would do a 50% water change and then another within a few days. For algae growing on plants and other decorations, try the hydrogen peroxide treatment mentioned above under the section on Brown Algae.

If you don’t mind occasionally finding small white eggs around the aquarium, you can stock a few nerite snails in your aquarium. They are algae-eating machines.

Blue-green Algae (Cyanobacteria). Source:

Blue-Green Algae (Cyanobacteria)

Description: Blue-green (BG) algae appears as a slimey, blue-green slimy mass on surfaces and floating at the top of the aquarium. It often forms sweeping sheets of material that can quickly coat every surface. BG algae does not have to be exclusively blue-green in color. It can be green, red, brown, or even black.

Blue-green algae is actually not a form of algae but a kind of bacteria. Ancient cyanobacteria are thought to be the precursor of all photosynthesizing organisms like plants and algae. The theory is that early single-celled organisms consumed some cyanobacteria cells, and instead of digesting them, the two formed a kind of symbiotic relationship. Over time the cyanobacteria changed into what we now call chloroplasts. Chloroplasts are the organelles in the cells of plants that carry out photosynthesis. As fascinating as that may be, we do not want it in our aquariums!

Cyanobacteria is a kind of nitrogen-fixing bacteria, which means it breaks down compounds like nitrates into nitrogen gas in order to produce energy. In small, controlled amounts this can actually be beneficial in a soil substrate, but more often then not it gets out of hand.

Threat Level: Potentially Harmful to Pets – Cyanobacteria can produce toxins that can build up in an enclosed environment. These can be harmful or deadly to fish and invertebrates.

Cause and Prevention: An outbreak of cyanobacteria can be caused by excessive light, an abundance of nutrients from food and waste, and anaerobic conditions. Anaerobic conditions can be found within the substrate and in “dead spots” at the surface of the substrate. These dead spots are caused by poor water circulation. An overabundance of food and fish waste tend to produce a lot of phosphates. In a heavily planted tank, you may be experiencing an imbalance of nitrates in the water. Plants use nitrates for their own processes and growth. A bloom in cyanobacteria quickly depletes the levels of nitrates in the water, weakening your plants. The plants then cannot out compete for other resources. Cyanobacteria capitalize on this in order to form a bloom.

Treatment: You can start by physically removing as much of the bacterial mat as you can. Then do a heavy water change of about 50% or more to try and reduce the excess waste in the water that the bacteria are feeding on. Then, for a planted tank, you can dose fertilizer high in nitrates and other nutrients to give your plants a boost, and help them to out compete the cyanobacteria. You  may see a bloom of greenwater or some other algae, but this is better to be dealing with than cyanobacteria.

Next you should try to increase the flow of water throughout your aquarium. Try repositioning the outflow from your filter or trimming plants and moving decorations to allow water to flow more freely. This is to eliminate those dead spots mentioned earlier.

Finally if the cyanobacteria persists you can try dosing with an antibiotic like Erythromycin. Since it is a type of bacteria, it should respond to this kind of treatment. I would recommend before using a treatment like this that you research your plants and animals to make sure they will not be negatively impacted.

Staghorn Algae

Description: Staghorn algae gets its name from it’s similarity in appearance to the antlers of deer. It can come in different colors like green, white, gray, or even shades of blue and purple. It is often seen growing on the edges of leaves and other surfaces in the aquarium. Most of the usual algae eaters don’t eat it, so it will need to be removed manually or chemically.

Threat Level: Mostly Harmless – This algae is more or less harmless and doesn’t produce any toxins. It’s big negative is its appearance. Like most algae, aquarists generally don’t like to see it growing in wispy clumps in their aquariums. I happen to think it looks kind of interesting. 

Cause and Prevention: Staghorn is generally caused by an excess of fertilizers/nutrients, especially iron.

Treatment: Staghorn algae can be treated the same way as Beard/Brush algae. You can begin by trying to remove as much as possible manually. Then treat the site where the algae was growing with a weak hydrogen peroxide solution (a ratio of 4:1, favoring water). If it is growing on a piece of decoration like a rock, piece of wood, or castle that you can easily remove, you can soak it in a container of the peroxide solution for a few minutes and scrub with a clean toothbrush. Any dead algae leftover from the peroxide treatment can be eaten by fish, shrimp, or snails.

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