1-03-2021, 20:47
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Red algae (Rhodophyta) and methods of combating them

Red algae, often referred to as red slime algae, are not actually algae - they are cyanobacteria. Cyanobacteria, often considered an evolutionary link between bacteria and algae, are some of the oldest life forms on Earth, dating back at least 3.5 billion years. These organisms produce oxygen as a by-product of photosynthesis and scientists believe that without this microscopic organism, there would be no blue skies on Earth.

The growth of red slime algae is a common problem in marine aquariums and must be dealt with to ensure a healthy environment for your aquarium plants and animals.

Identifying red mucus algae

Cyanobacteria are often called blue-green algae. However, only about half of these organisms are actually blue-green. Most of the forms found in saltwater fall into the range of other colours, including:

  • Blackish green to blue-green;
  • Orange-yellow to reddish brown;
  • Dark purple to completely black

Red slimy algae begin as small spots and, if no action is taken, spread rapidly throughout the aquarium.

Causes and solutions for red mucus algae

The excessive growth of red slime algae is usually caused by light or nutrients in the water. These are two ingredients that all algae need to grow. When trying to find a cure, try each solution in turn. Otherwise, when the problem disappears, it will be impossible to know where it originated and which solution helped fix it. Start with one remedy and see what results you get. If one remedy doesn't work, try another and so on until the problem is solved.


The use of unsuitable light bulbs, lack of maintenance and extended lighting hours are factors that can lead to all sorts of algae problems. Although these organisms do well in the 665 to 680 nanometre (nm) wavelength range, they are also quite active in the 560 to 620 nm range.


Only use lamps that are designed for use in the aquarium;
Turn on the lights for no more than 8-9 hours per day, depending on the needs of the aquarium inhabitants;
Try different types of lamps to increase the light intensity and optimize the spectral characteristics of the light in the aquarium.


Phosphate (PO4) and nitrate (NO3) are the main food sources for red and other slime algae. Phosphates usually enter aquariums using unfiltered fresh tap water as well as sea salt mixtures, activated carbon, food, etc.


  • Only use settled water and high quality sea salt;
  • Be aware of other elements that contain elevated levels of PO4 in their composition.

Red algae (Rhodophyta) and control methods
Dissolved organic compounds (DOC)

The accumulation of excess DOC in the aquarium causes problems with nitrate (NO3). However, nitrates can also be eliminated in the same way as phosphates and, as it is an end by-product from the nitrogen cycle, they can naturally reach high levels due to lack of proper aquarium maintenance. 


  • Practice systematic aquarium maintenance: keep the substrate clean, remove uneaten food on time, wash and change filter or absorbent materials (filter filaments, cartridges, bio-wheels, sponges, charcoal) regularly, do partial water changes regularly.
  • Use a skimmer.
  • Add a few algae/detritus-feeding hermit crayfish, one or two real crabs, shrimps or other good "wipers" that sift through the substrate.
  • Avoid adding new fish and shellfish if the aquarium is experiencing PO4 and NO problems Also, avoid any water changes and any preventive substrate or filter cleaning activities, except replacing dirty filter media, until the aquarium has completely completed the nitrogen cycle. Since red slimy algae do not attach well, they can easily be cleaned and removed with a light siphon, and larger floating pieces can be removed with a net.

Combating red slime algae with additives

Although cleaning the aquarium and following proper maintenance procedures will not bring immediate results, you can use one of a number of additives to quickly resolve the problem within 1-2 days. Keep in mind, however, that many of these treatments only kill the slimy algae, but do not get rid of the underlying problem that causes them to reappear and further accelerate their growth.

Since cyanobacteria are a type of bacteria, many of the currently used supplements are antibiotics, which are drugs that can weaken or completely destroy the beneficial bacteria in the aquarium biological filter. Use these treatments carefully!

The test may deceive

Since slimy algae consume nitrates, the readings often remain normal when aquarists carry out nitrate tests. Do not be fooled. If you had temporarily removed the algae before using any other solutions, you would probably have noticed an increase in nitrate levels in the aquarium. Nitrates have actually been there all along, but they are undetectable when the algae feed on them, giving the impression that they are under control. This also applies to many other forms of algae.

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