The first thing you’ll likely have learned about Cyanobacteria (“Cyano”) is that it is not an algae, but is instead a type of bacteria. This means that most algaecides won’t kill it, and that it has a specific niche where it can inhabit certain environments.



Some basic facts about Cyanobacteria and things to remember:

  • There are several different strains of Cyano, with each strain having different arrangements of cells such as the common filamentous strain (Oscillatoria) to the unusual rectangular colonies formed by Merismopedia Cyanobacteria.

  • Cyanobacteria are photosynthetic, meaning it relies on light to live. They use Chlorophyll A, Phycocyanin (which gives it the classic blue-green tint) and some Phycoerythrin. The absorption wavelengths for these are given below, however it is important to note that Cyano thrives on green-yellow light.

  • Cyanobacteria are anaerobic, this means that they thrive in areas with low flow and thus low oxygen.

  • Colonies can double in size within a day’s time as they spread very quickly. This means that while quick methods such as algae killers and antibiotics (particularly erythromycin) may be suggested, Cyano can quickly bounce back after treatment has finished. This makes it essential to treat the cause, rather than to use “band-aid” solutions.


filamentous cyanobacteria

Above: Filamentous Cyanobacteria


  • No freshwater fish will eat Cyanobacteria. It’s also important to remember that while fish may eat certain types of algae, the addition of them adds to your bioload and overall nutrients, which can further feed other types of algae.

  • Like algae, Cyanobacteria requires nutrients to survive. A heavy infestation of Cyano may indicate a too high a bioload, or too few water changes.

  • The death of a large portion of Cyano can lead to a sudden increase in nutrients (such as Nitrates) in the tank. It’s important to keep up water changes to prevent nutrients from rising too high. Make sure that your filter is large and healthy enough to deal with the increase of Ammonia, or an Ammonia or Nitrite spike may develop.

  • Changing one variable at a time may be preferable to allow for you to figure out what the general cause is. Changing too many variables at once may lead to confusion and even outbreaks of other kinds of algae.


Cyanobacteria Wavelength

Above: Some of the wavelengths of light which Cyano may absorb.


Here are several steps that can be taken to reduce the growth of Cyanobacteria:

  1. Water changes, water changes, water changes. Cyano will be thriving on the amount of nutrients and organic build-up in the tank. The death of Cyano will also increase these nutrients, and water changes are the most efficient way to remove the excess. It is also very useful to physically remove the Cyano which will easily collect in the siphon, making water changes even more important for the reduction of Cyanobacteria.

  2. Lighting is very important and has many factors. The first factor is lighting hours. Often, it’s not necessary for lights to stay on longer than 8 hours a day - even 5-6 hours may be preferable. Try to keep your lighting hours within the hours of the ambient light around your tanks to prevent the overall lighting from being on for too long.

  3. Lighting quality is the second factor. As mentioned above, Cyanobacteria thrives in green-yellow light, which plants need very little of. Poor quality lights are high in the green spectrum due to green being the brightest colour. Good quality lights may look no brighter, due to the lack of green-yellow, however they are typically better for plant health and reduce undesirable algae growth. It is also preferable to check the recommended replacement time for your lighting, as some lights (such as Fluorescents) change their spectrum as they age and may need changing every 6-12 months.

  4. Ambient light can have a significant effect on what happens in your tank. Direct sunlight, especially during summer, will grow all sorts of algae along with Cyano. For tanks with a lot of sunlight, it may be worth blacking out the back and sides of the tank. For tanks outside, covering the tank during day time may even be necessary.

  5. Increasing water flow and surface agitation can be used to increase the oxygen levels in your tank. As mentioned above, Cyano thrives in areas of low flow and low oxygen. Low oxygen levels can cause problems in many fish, however be sure to not increase the flow higher than what is comfortable for your fish. Fish such as Gourami and other air-breathers are adapted to low-flow environments and may struggle if the flow is increased.

  6. Nutrient levels are a big problem. While water changes are often the first step to reducing these, other methods may include using Chemical media, reducing your bioload, and using plants. Plants are very useful in tanks for so many different reasons, but especially for reducing nutrient levels. Without anything removing nutrients from the tank, there is nothing to out-compete organisms such as Cyanobacteria. Fast-growing plants such as Ambulia, Cabomba, Hygrophila polysperma, Indian fern, Stargrass and Rotala rotundifolia are all useful for uptake of nutrients. If Submerged plants are not suitable, Emersed+ plants may be preferred.

  7. A 3-4 day blackout can be immensely useful for “knocking back” the Cyanobacteria. A blackout includes keeping the tank light off for several days and, if possible, covering up the sides of the tank to prevent ambient light from entering. While this isn’t a permanent fix, a blackout can be implemented to kill off a large portion of the bacteria while other variables such as lighting are then changed. It is preferable to do a water change after this blackout to clean up any dead bacteria and to help remove any excess nutrients that the dead bacteria has created.


Antibiotics: To use or not to use?

Whilst this is a personal decision, and some do recommend the use of Antibiotics (ABs) against Cyano, we personally recommend against it for several different reasons. Antibiotics have to be used with a lot of caution as they have consequences for your fish - the effect of ABs lessens when used more than once, and the risk of Antibiotic resistance in the tank increases - along with consequences to the environment as the tank’s waste water goes into our sewage system and affects Antibiotic resistance in humans. Along with this, several Antibiotics including erythromycin, can kill the beneficial bacteria in your filter which may cause Ammonia, Nitrite and Nitrate spikes, leading to stress and disease in your fish if not dealt with correctly.

As Cyanobacteria often bounces back if the cause is not corrected, we believe a tank blackout is just as effective as Antibiotic treatment and has less consequences for fish, us humans and our environment.



Cyanobacteria were one of the first photosynthetic forms of life on Earth and are found in almost every aquatic environment on the planet. This makes the bacteria quite diverse, so it’s known to be difficult to overcome. Like Algae, a small amount of Cyano is a normal part of your tank’s ecosystem - however, too much can cause problems for the health of your tank. Cyanobacteria requires patience and effort, but once the balance is found, it is well worth it. If you have any questions about eliminating Cyanobacteria, feel free to contact us, or have a look at a local forum such as the FNZAS for opinions and advice from other hobbyist fishkeepers.