How to Remove Red Algae in Aquariums

How to Remove Red Algae in Aquariums

Red algae refer to the group Rhodophyta, which includes notorious types of algae such as Black Beard algae and Staghorn algae.

The problem though is that these common names do not necessarily refer to specific algae species. So, in some cases, other species of Red algae may appear in our aquariums, which may have similar characteristics (and may even look alike) but require different methods of control.

Despite the fact that algicides have proven to be quite effective against Red algae, in general, but even these treatments may fall short, making the management of these algae particularly challenging.

That’s why it is important for us as everyday aquarium hobbyists to educate ourselves about various types of algae. By gaining knowledge about them, we can better understand what to expect and take preventative action to stop their occurrence and proliferation in our tanks.

Interesting fact: The origin of Red algae has been a mystery. Initially, they were classified as plants and later recognized as the earliest eukaryotic organisms. Phylogenetic analyses strongly suggest that red algae emerged early in evolution before the lineages that gave rise to plants, animals, and fungi.

What Are Red Algae?

Rhodophyta, commonly known as Red algae, are a diverse group of marine and freshwater algae characterized by their red, purplish, and black coloration.

All representatives of Rhodophytes possess chloroplasts that produce chlorophyll. It is the same chlorophyll found in other plants, but its green color is masked by an auxiliary photosynthetic pigment called phycoerythrin, which is a unique pigment present in Red algae.

The pigments phycobilins, such as phycoerythrin, and phycobilins absorb blue light and reflect red, giving many Red algae a distinct bronze-red hue (noticeable when held up to the light or when they are exposed to acetone or alcohol) and a black appearance under normal lighting.

In a small group of freshwater red algae, including Audouinella, the level of phycoerythrin is reduced. As a result, the “black tufts” may sometimes appear gray or even slightly greenish.

Description of Red Algae

Red algae look like a cluster of strands. Its color is more of a dirty gray, brown, or black, depending on the species and environmental conditions.

These algae reproduce via spores, most commonly asexually, while sexual reproduction was also recorded.

In aquariums, this type of algae does not have a specific location. It can be found on plant stems, especially on their bends or on the tips of leaves, as well as on various decorative objects such as stones, caves, artificial plants, etc.

Difference between Species of Red Algae

Lots of aquarists do not even know about different types of Red algae, which often leads to difficulty in selecting the most effective methods to combat them.

Actually, this is not surprising! 

Note: According to various sources, there are currently between 5,000 to 10,000 described species of Red algae. Almost all of them are marine algae and around 200 are freshwater species.

For example, at present, the genus Audouinella includes over 80 species of algae, which also include Black Beard algae. Even scientists recognize that these species are still poorly defined and sometimes based on unreliable characteristics.

For example, some Red algae species may look very similar, especially, in the beginning stages of their growth. However, as they continue to grow, it will get much easier to differentiate them.

For example, compared with the Black beard algae that grow up to 2 inches (5 cm) and elongate, creating a fringe of dark threads, there are algae that form brush-like clusters from the beginning and grow no more than 0.8 inches (2 cm) in length.

Causes of Red Algae

I would like to start off by saying that it is quite challenging to prevent the appearance of Red algae in aquariums as their spores are present basically in all aquariums and can rapidly proliferate under certain conditions.

Yes, you heard it right. Our aquariums harbor a vast array of microorganisms and algae species. It’s merely a matter of providing the right conditions and giving them time to thrive and develop.

That is why even well-maintained aquariums with lots of plants can sometimes encounter outbreaks of these algae.

Nevertheless, let’s take a closer look at what factors primarily influence the growth of Red algae. The main causes of their appearance include:

  • high organic concentration in the tank (overstocking, overfeeding, large pieces of driftwood),
  • poor water filtration,
  • insufficient oxygen in the water or excessive aeration,
  • CO2 disbalance,
  • infrequent cleaning of the aquarium,
  • insufficient water changes.

The growth of these algae in an aquarium is typically caused by a combination of these factors.

However, if we look for a unifying factor, it becomes clear that the amount of organic in the water serves as the main catalyst for their occurrence.

Are Red Algae Harmful to Plants?

Yes, Red algae are bad for plants. Generally, plants that grow slowly and/or have broad leaf blades are the first to suffer (for example, Anubias, Amazon Sword, Java fern, etc.).

During the initial stage of growth, Red algae start to accumulate along the edges of plant leaves. However, as they continue to grow and spread, they will progressively cover a larger surface area of the leaves, hindering the plants from receiving the necessary light for photosynthesis. As a result, the affected plants will suffer from nutrient deficiencies, become susceptible to diseases, and ultimately decay.

If left unchecked, these algae will rapidly proliferate to the extent that the only viable option might be to completely eliminate all the plants in the aquarium.

Are Red Algae Harmful to Fish or Shrimp?

No, Red algae are not harmful or dangerous to the fish, shrimp, or other animals in your aquarium. So no, these algae will not hurt them.

Nonetheless, indirectly, these algae can have a negative impact, which can manifest in the following ways:

  • Excessive growth of these algae may ultimately cause an imbalance in nutrients and will lead to an unsafe environment for animal species.
  • If you keep fish species that love to eat plants will find plants covered by these algae unattractive.

Methods of Eliminating Red Algae

As Red algae belong to the group Rhodophyta, it is worthwhile to take into account the methods I previously described in articles about Black Beard algae and Staghorn algae.

However, it is important to keep in mind that these methods may not always be effective since they are still distinct species (or even genera) of algae.

It is also important to note that not all methods are universally effective for every tank. Furthermore, the efficacy of each method can vary depending on the extent of the infestation and the specific setup of the tank.

Generally, Red algae are quite tolerant of high levels of pollution, acidity, and thrive on dissolved phosphate and nitrates.

Chemicals (Glutaraldehyde or Monolinuron)

Although I don’t strongly support the use of various chemicals against algae, in some cases, there may be no alternative but to resort to drastic measures.

It has been discovered, for instance, that this specific kind of algae is relatively susceptible to Glutaraldehyde (C5H8O2).

Glutaraldehyde diluted with water is often sold as an alternative to carbon dioxide gas injection for aquarium plants. Unfortunately, due to its high potency, pure Glutaraldehyde may harm some plants (such as Ceratophyllum spp, Vallisneria, Marimo moss balls, Subwassertang, Elodeas sp., Crystalwort, etc.).

Note: As a disinfectant, Glutaraldehyde is often used as a disinfectant to sterilize surgical instruments.

How to Remove Red Algae in Aquariums - Easy Carbo and Flourish ExcelFor example, Easy Carbo and Flourish Excel are variants (isomers) of glutaraldehyde (C5H8O2), but it is slightly less reactive compared to the original formula.

Seachem Excel  – link to check the price on Amazon.

How to Remove Red Algae in Aquariums Tetra productsMonolinuron (C9H11ClN2O2) is also an algicide but with lesser toxicity than others. Examples of products containing this algicide:

  • Tetra Algetten,
  • AlgoStop depot,
  • Tetra Algizit,
  • Tetra AlguMin

How to Remove Red Algae in Aquariums tetra compareNote: You can see from the graph that some product start working right away but just for a limited period of time. Others require more time and have a milder effect.

As a recommendation, it is better to turn off filtration for 30-60 minutes after treatment to increase its effect.

Important: Do not overdose glutaraldehyde! Read instructions! For example, the toxicity threshold of Excel for invertebrates and fish is relatively low (>5mg/L), you can kill your animals.  

Improving Filtration

Weak aquarium filter, improper organization of water flow, and highly contaminated filter elements.

If you notice the presence of Red algae (or basically any other type of algae), it is highly likely that there is an abundance of organic matter in your water.

So, even if your water appears crystal clear, it doesn’t necessarily mean there are no micro-particles present. For instance, driftwood can contribute a significant amount of organic matter. That’s why it’s important to enhance or improve your filtration system.

Cleaning, Vac Cleaning, and Water Changes

It is recommended to perform a weekly substrate vacuuming while doing a 30-40% water change. We need to do that to remove detritus and any organic matter that may be stuck in the substrate.

It is crucial to maintain a close watch on the situation by regularly testing for phosphates and nitrates as algae heavily relies on NO3 and PO4 as nutrients.

Additionally, ensure utmost cleanliness in the chambers of mechanical filters, including pre-filters, even if you have efficient canister filters in place.

Bleach decorations and rocks for 10-20 minutes. Wipe thermometers and other glass equipment.

Remove driftwood from the tank if it is possible (even if you boil, bleach and cure it, it will still decompose over time which is the main problem with these algae).

Lighting control

Red algae thrive in well-lit conditions. Thus, adjusting the lighting duration and intensity can help discourage its growth. Consider reducing the lighting period or investing in a timer to regulate the light cycle.

Reduce the duration of light exposure. Limit the photoperiod to a maximum of 10 hours regardless of the circumstances.


Red algae thrive on chemically bound CO2 in the form of bicarbonate [HCO3-].

In aquariums where CO2 is not supplied and the water is hard with a high pH, the majority of carbon exists as bicarbonate [HCO3-].

Conversely, in aquariums with CO2 supplementation, resulting in a pH level of around 7.0, most of the carbon is present as CO2. As a result, the presence of Red algae is significantly reduced in such CO2-injected tanks.

Regulating the Phosphates and Nitrates

Nitrates and phosphates naturally increase as wastes are broken down in the aquarium. If the tank is not properly maintained, it will continually rise, cause an imbalance, and contribute to algae growth.

Nitrates and phosphates can originate from various sources in an aquarium ecosystem. These include:

  • fish waste,
  • uneaten food,
  • dead animals,
  • decomposing plant matter,
  • fertilizers,
  • pH & KH buffers, etc.

Manual removal

To be effective, this method should only be done combined with others mentioned above.

This species of algae is quite resilient and firmly attaches itself to its place of growth. If the algae have completely infested the leaf, it is most likely much easier to simply remove it.

  • Trim the affected plants
  • Try to physically remove as much algae as possible.

Use algae scrapers or a toothbrush, for that.

Ineffective methods

Of course, every aquarist has their own experience, what works for one person may not work for another.

However, it has been shown that some methods for eradicating this specific variety of Red algae have been quite ineffective. It is probable that adjustments need to be made in their application specifically for dealing with these algae. So, if anyone has any additions or observations, I would be happy to hear them.

  • Blackouts

Darkening the tank for 3 days did not help me but it was pretty effective against Black beard algae. 

Despite the fact that Red Algae (Rhodophyta) are photosynthetic and autotropic (needing only carbon dioxide or carbonates as a source of carbon and a simple inorganic nitrogen compound for metabolic synthesis). Some species of Red algae can tolerate blackout for long periods. 

  • Hydrogen Peroxide (H2O2)

Personally, I consider this method to be not ineffective because it requires a high concentration of Hydrogen Peroxide (up to 50 mg/L for 20-30 minutes) and preferably a separate container to bath. What is the point of going through all that trouble when you can simply use bleach in this case?

Important: This concentration can kill your fish, snails, shrimp, and even beneficial bacteria. DO NOT do that in your main tank!

  • Algae eaters

There are some fish and shrimp species that may eat Black beard algae but with some other types of Red algae, I did not have any luck with any animals (fish, snails, or shrimp). They refused to eat it.

Although I admit that it could be particularly my situation and animals (Amano Shrimp, Nerite snails, Siamese Algae eater, and Mollie) were not really hungry, so I need more tests.

At the same time, once the Red algae start decomposing, Amano Shrimp, Siamese Algae eater, and Mollie will readily eat it.

Preventive Measures

Like all other species of algae, Red algae spread out when they find a favorable environment. Therefore, based on what we know about this specific kind of algae, measures can be taken to reduce the risk of its occurrence in the tank.

  • Feed control. Do not overfeed your fish.
  • Cleaning. Regularly remove excess fish food and fish waste. Clean the substrate monthly. Do regular water changes.
  • Use fast-growing plants. They will help you to reduce nitrates and phosphate from the water.
  • Quarantine new livestock and disinfect new plants.
  • Control your water parameters.

In Conclusion

Under certain conditions, Red algae can spread rapidly, causing significant damage to aquatic plants.  

The presence of these algae in your aquarium is a sign of an imbalance and an excessive amount of organic matter in the water.

To address this situation, controlling the quantity of food and fertilizers is a good place to start. You should also keep an eye out for any driftwood or rotting plants that might be a contributing factor.

By combining routine water changes, manual cleaning, and the application of algicide products, you should be able to eliminate these algae from your aquarium.


  1. Necchi Jr, Orlando, and Marcelo R. Zucchi. “Systematics and distribution of freshwater Audouinella (Acrochaetiaceae, Rhodophyta) in Brazil.” European Journal of Phycology30, no. 3 (1995): 209-218.
  2. Huisman, John Marinus. Marine plants of Australia. University of Western Australia Press in association with Australian Biological Resources Study, 2000.
  3. World Health Organization (2009). Stuart MC, Kouimtzi M, Hill SR (eds.). WHO Model Formulary 2008. World Health Organization. pp. 323, 325. hdl:10665/44053. ISBN 9789241547659.

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