How to fight a Cyano bacteria problem

The german aquarist Bernd K. just wrote a very interesting summary about one of the biggest problem in saltwater aquariums: Cyano bacteria. Special thanks – we’re happy that Bernd allowed us to share this on! 

The basics for Cyano phenomenon

A frequently recurring misunderstanding: In older literature, but also in today’s discussions, the aquarist speaking about red or green slime algae or cyanobacteria, but cyanos are, although they almost all do photosynthesis, bacteria due to its lack of a nucleus. Therefore, the most distinguished fighting approaches against algae are ineffective!

They are often mistaken with diatoms and dinoflagellates. Therefore an accurate determination is essential (also see other posts, such as FAQ alga).

 Why are there now so many contradictory statements about fighting cyanos?

The main mistake that still occurs still at knowledgeable aquarists, is the false assumption that we are dealing with only one type of cyanobacteria in our aquariums, and thus we only need to handle a few causes for their proliferation. Cyanobacteria are among the oldest living ceatures on earth, with at least 2000 different species and over 250 maritime forms (for more detail interested: The genera Anabaena, Nodularia, Oscillatoriales, Merismopediacea and Microcoleus). There is also a extremely high variability and mutation readiness within these species, so that the taxonomic classification, supported by new possibilities of molecular biology sequences analysis, is in a continuous flow today. This wide variability allows cyanobacteria to adapt to their habitat and it’s special conditions quick and efficiently.

These circumstances, so that not just  one form of cyanobacteria appears, despite the same appearance a completely differentiated pathogens and different causes may exist. Thats why we are not always successful with the same method of combating!

What makes things even more complicated, that each of our tanks has a completely different microbiological condition. Cyanobacteria are constantly latent existing. Their potential for proliferation is also in a direct relation with the microbiological conditions. Scientific studies are covering only some aspects, but only few considers the complex interplay in our tanks. It follows that no one completely overlooks the relationships today.

Generation of films and related problems

In good running reef tanks, there developed a stable microcosm of microorganisms and zooplankton after some time. If these are in equilibrium, each organism performs its specific function. However, in this well-established system disorders may lead to sudden Cyano outbreaks. With the outbreak of cyanobacteria, they link together into mats by sugar and protein fibers that can cover everything in the aquarium. The ugly appearance is the least of the problems; more problematic is, that they could cover corals, especially the dispensary of strong neurotoxins. These toxins, but also an emerging lack of oxygen by microbial degradation processes when Cyanos die off in larger amounts, can lead to death of the whole aquarium stock.

Therefore, it is of great importance to keep the mass of cyanobacteria in your tank in a tolerable degree! You never can completely eliminate Cyano, at least they would appear again in no time.

Possibilities to handel Cyano bacteria

Because of the above mentioned points, there can and will never be a general patent. Finally, it remains on a number of trial and error actions.  The main reason for many failures is unsystematic activism. There are all sorts of good and bad advices which bring no effect in a short time. So you try another hint and at the end nobody knows, what action leads to a special effect. Oftentimes, this means messing whole tank biology with unpredictable consequences.

 Therefore, it is very important to do all activities with calm and patience, to document it, to observe the impact as positive or negative on the growing of Cyano and to keep longer intervals betweens each intervention. Always keep in mind: Nothing works immediately!

Early dammning of Cyano

The timely containment is very important at the beginning, to prevent a large-scale expansion. This is done by regularly exhausting smaller deposits. If once a week is enough to keep Cyano in a tolerable amount, just replace the water with your weekly water change. If there’s need to do it daily, the water is filtered through wadding and returned into the tank.

Also good is taking out the shreds of Cyano with a fine aquarium net. Of course, this needs a certain effort, especially because it must be continued for weeks to months. However, this already prevents possible damage on corals and inhabitants, especially the development of specially adapted Cyano-mutations in your tank. This action itself, sustained over time, can already lead to the collaps of the plague.

Nutrient ratios

An oversupply of nutrients, nitrogen and phosphates or their imbalance is mostly regarded as the main cause of explosive outbreaks. This is mainly based on the study of such phenomena in the ocean, such as the so-called “red tide”. But is this the actual reason in our tanks?

The are recommendations at internet forums that advise to renew the ground as an alleged phosphate deposit, but usually without lasting success.This theory is based on the idea that cyanobacterias, in contrast to their competitors, fix atmospheric nitrogen or reduce ammonium, and thus at the presence of phosphate can compensate a lack of nitrogen, which gives them a competitive advantage. Very nice, but only some of the occurring Cyanos have heterocysts for the nitrogen fixation! Besides this fact there are only few tanks that have a defecit of nitrogen. Usually the measured NO3-values ​​get misinterpreted as a nitrogen limitation. Finally, nitrogen is not only existing in its form as nitrate!

There are observations from the Baltic Sea, where outbreaks frequently occur in times of low phosphate concentrations. At connected tank systems it can be observe sometimes, that the Cyano plage occurs in one tank, but at another tank from the same watersystem no infection appears. In own experiments I could verify that cyanos even continue to live and grow in glasses that were filled with clear water.

The biologist Claudia Wiedner has also been shown in a dissertation, that experiments with Oscillatoriales phosphorus can be excluded as a control variable for the propagation rate. Similar findings resulted from her investigations in terms of nitrogen. Additional nitrogen doses did caused only small increases in biomass compared with a lack of nitrogen. At a high nitrate concentrations even a decrease in biomass was observed. Conclusion, you should bring in the distorted nutrition ratios at optimal areas, but an elimination of cyanos is not automatically guaranteed.

 Pure water forms

Due to the different reactions of Cyanos on the nutrient conditions, the theory of so-called pure water Cyano forms has formed. Whether it can solely originated on the nutrients remains an open question. It is striking that Cyanos develop good at a high redox potential or near the return of the skimmer, and even sometimes directly as a film on just slightly moving water surfaces. An increased oxygen supply as a promoting cause might considerable aswell.

As part of the recommended systematic search for causes one should pay attention to these urgent issues: Are Cyanos getting more at high or low nutrient levels? How do they react to changes of water and on stronger or weaker water changes or changes at the skimmer? In this context, a change of the salt mixture could be tested. Even small differences in the composition cause success. And you’ll never exactly know why.  If you increase water changes on the one hand without success, try to stop them completely for a while. If you have a massive outbreak of Cyano, stop additions like trace elements or amino acids as well as coral food at all.

Physical processes

It is remarkable that there are more reports of Cyano outbreaks coming with the technical revolution of powerful flow powerheads. Indeed, there is a study by the University of Bremen on tests for antibody formation in cyanobacteria.Indeed, there is a study by the University of Bremen on tests on the formation of cyanobacteria. Thus, physical processes such as vortex flow may lead to local accumulations or levels of its kind. Particularly the increased flow may promote the development of extracellular polymeric substance – the formation of a biofilm. Therefore it’s recommended to change or reduce the flow in your tank as a single test. (Note: I run my studded with heavily overgrown coral pool with very low laminar flow!)

Salinity, alkalinity and light

A growth-promoting salinity was identified only at low values ​​of 15-18, far below the usual aquariums values​​, a higher alkalinity tended to be slightly stimulating Cyano.

Because of the wide adaptability of Cyanos, this resulted in no significant fighting approaches. The same applies to the macro elements such as calcium or magnesium. Necessary for photosynthesis of Cyano is light. There was a strong variation of the concentration of the light-harvesting pigments by 4 – 5 times between high and low light determined, which again demonstrates the strong adaptability of Cyanos. Cyano tend to grow better at low light intensity, so it makes a lot of sense to replace old tubes or HQI. The light quality (spectral), however, has no seignificant effect.

A complete darkening of the tank makes no sense. Until the total lack of light Cyanos brought to die off, all other inhabitants will be dead, too.

Bacteria competition and phages

For me, the most important reason for the establishing of large quantities of cyanos seems to be the lack of competition from other microorganisms. It’s probably less a food competition, as to compete for “free seats”. It can be observed regularly that just uninhabited places, eg freshly dead coral branches, cyanos especially like to settle. This is due to the widely differentiated defense mechanisms of many microorganisms, they keep the competition away. Therefore, it makes sense to populate a tank very soon. On good living and healthy coral rock or other organisms such as corals you can find cyanos seldom. In order to promote competition conditions, bacterial supplements are used often. However, the successes are quite different. This may vary due to the different composition of the bacterial mixtures.
Which bacteria are suitable? Actually I don’t know, and this may even apply to distributors of such products. In this case you are dependent on the method of trial and error.

Whether there are any other bacteria that fight cyanos their living environment? Further microorganisms such as algae, fungi, protozoa, etc might be imaginable. In these situations bacteriophages take a special role. Bacteriophages are viruses that attack certain bacteria. In medical trials we make use of this attribute to combat so-called multi-resistant germs to use, where no antibiotic takes effect.
These phages get isolated from sewage with an enormous effort; because they appear frequently where the concentration of bacteria is high, thus experiments were close to try it as well in aquariums. There are actually some successes, but broad results are not available yet. In my opinion there is no reason why not to try a few ml of water from a sewage drain. The introduction of fish diseases, etc. needs not to be worried, because that sewage does not come from fish waters. But the own fingers do not necessarily take a bath in it.

However, an old method is extraction of soil. It brings a number of beneficial microorganisms, not only bacteria, and even phages to the tank. Take a few spoons of unfertilized garden soil, if possible from different locations, if available some good compost, and mix it with water in a jar. After that, let the soil depose, and give the clear liquid portion on top into the tank. Unfortunately a success assurance can’t be promised with this approach. It depends a lot on the type of organisms accidentally introduced, but it does have chances of success and it costs not even a cent.

Cyano remedies

However, any additions available in the market often cost lots of money and don’t actually help. To make it clear; I’m an absolute opponent of those. From time to time they appear accompanied by success stories. After some time it gets very quiet. They have helped, sometimes quite successfully, the purse of the distributor. But there are far less frequently reported stories.

If these additions are based on a carbon source (for example, there were probably simple dextrose solutions sold expensive) it promises to be a “feeding” of competing bacteria in the tank. Meanwhile, due to own negative experiences but especially the large number of negative observations of other people, I’m not a fan of carbon in general.  I’m sure that not everyone shares my opinion. Reports on the later outbrake of dinoflagellates, undesirable macro-algae outbreaks or even only Cyano are oftentimes mentioned.

What you definetely should never use are bactericides, wheter on the basis of silver or other disinfectants. They definetely help against cyanobacteria, but that’s not all. First, the mass extinction of all bacteria can mess up the water and after that, everything in your tank dies.

The dumbest thing you can ever make is the use of antibiotics. Sometimes sold as “without any antibiotics” they came mainly from the U.S. and Netherlands. For good reason, the use of these is prohibited in Germany. They cause damage to the the environment, even the health of the user may in some cases directly threaten, but destroy the biological population overall. The antibiotics bacteria can not distinguish between beneficial and harmful bacteria. After using the Cyanos are actually completely dead but with them also their competitors. And after that, Cyanos are the first appearing again – this time without any competitors.


With calm and patience almost all aquarists handled the Cyano problem. It takes time and has to be done very structured. Perhaps, all above mentioned helps you to fight against Cyano. Good luck!

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