Blue-green algae (Cyanophyta), Schizophyceae, Myxophyceae.
(Cyanobacteria, Blue Green algae (BGA), Slime or smear algae)
Description. Blue-green algae (Cyanophyta), Schizophyceae, Myxophyceae – how many different names this group of the most ancient autotrophic plants has received from researchers. Blue-green algae are actually not algae, but colonies of bacteria, which helps to understand how to get rid of them. They are called algae because they live by photosynthesis. Cyanobacteria, or blue-green algae (Cyanobacteria) are unicellular, filamentous, and colonial microorganisms. The average cell size is 2 µm.
They are distinguished by an outstanding ability to adapt the composition of photosynthetic pigments to the spectral composition of light, so the color varies from light green to dark blue. They are the main participants in water blooms. Their unique ecological position is due to the presence of two hard-to-combine abilities: photosynthetic oxygen production and atmospheric nitrogen fixation.
Blue-green algae perfectly extract all necessary elements from the environment, and in particular nitrogen. They independently synthesize all substances required 
for their development. This method of nutrition is called autotrophic. In addition, algae can also use ready-made organic compounds contained in water. Blue-green algae of the genera: Microcystis, Oscillatoria, Anabaena, are capable of mixed, photoheterotrophic nutrition, meaning they combine photosynthesis with the absorption of ready-made organic substances, which ensures their existence even in the dark and doubles their reproduction rate. Like red algae (Rhodophyta), blue-green algae have the photosynthetic pigment phycocyanin. Therefore, these types of algae compete with each other.
Blue-green algae, or Cyanophyta (cyanobacteria), include a group of approximately 2,000 species. They can be identified even with the naked eye by seeing a gelatinous mass, fine-fibrous growth, and “water bloom”. Blue-green algae form colonies of various colors, from bright green to bluish and brownish-green. By typical blue-green algae (genus Oscillatoria), aquarists mean a mucous, slippery bluish-green coating that quickly covers all aquarium equipment with a thick carpet, suppressing all life around. A colony can, for example, look like a blue-green sheet, long filamentous threads, or a hollow sphere. Many species of blue-green algae can spread to new places by forming motile threads. These blue-green algae have a characteristic unpleasant odor.
In addition to such blue-green algae, there are other types of algae that are equally viscous and black or brown in hue. They are odorless, but they multiply at the same rate and form the same mucous growth. A feature of all blue-green algae is the secretion of abundant mucus, which is why their colonies resemble a gelatinous or mucous film with a rather strong unpleasant odor.
Blue-green algae cover glass and aquatic plants with a slippery dark-green coating. These algae slow down the development of green algae and some higher 
plants. Aquatic plants covered with these algae have slowed photosynthesis, poorly absorb minerals, and as a result become weakened and lag behind in growth. The development of blue-green algae on the substrate leads to stagnant phenomena and a decrease in the redox potential, because toxic products of their metabolism accumulate in the substrate beneath them: ammonia, nitrites, hydrogen sulfide, methane, etc. They shift the water pH towards alkalinity.
When cyanobacteria lose the ability to regulate their density, for example, during temperature drops or disruptions in oxygen exchange, they float to the surface and cause the so-called “bloom” of water. In this case, cyanobacteria release chemical substances that are toxic to other organisms and lead to their mass mortality. (Recall that cyan – CN4, a poisonous gas). Individual representatives of blue-green algae genera cause the death of Daphnia, tubifex worms, fry, and in cases of water “bloom”, i.e., rapid development of microscopic green algae, even adult fish. Recently, the toxic nature of substances released into water by blue-green algae has been established, which makes their presence in a limited aquarium volume especially undesirable.
Identification: Blue-green algae — Blue Green algae.
Causes. There are many reasons for the unexpected, at first glance, population explosion of blue-green algae. First of all, this is an increased content of low-molecular-weight organic substances in the water (amino acids, carbohydrates, etc.). An excess of organic substances in the aquarium appears, for example, due to the decomposition of uneaten food. Another reason is a slightly alkaline environment (pH 7.5-9.5).
Blue-green algae, like other algae, are very sensitive to the content of trace elements in the water, which include, in particular, certain metals: iron, manganese, zinc, copper, etc. Their content in the water depends on the pH value. In an alkaline environment, where the salts of these metals are poorly soluble, the necessary concentration of these elements is provided, the excess of which (in an acidic environment) is fatal to algae. The development of blue-green algae is also promoted by a low oxygen content in the water and a low redox potential value. Their appearance is evidence of a general disease of the aquarium as an ecological system.
Blue-green algae appear and begin to multiply rapidly in intensively aerated aquariums or (and) in the presence of an excess amount of organic compounds in the water. Cyanobacteria appear most often when there is an excess of phosphates [PO4] in the water and nitrates [NO3] = 0! That is, there is a major shortage of nitrogen [N] for plant growth. Most often, these algae begin to multiply under strong lighting on the surface of filter materials in external filters, in airlift tubes, and on the water surface, i.e., in places where, being in the aquatic environment, they have the greatest contact with air.
Intense lighting, especially of a certain spectral composition (sunlight), also increases the photosynthetic productivity of blue-green algae, which leads to their accelerated mass development. They also develop in slightly alkaline water (optimum pH 7.5-9.5), so their presence can in some cases (which should be avoided) serve as a biological indicator of the environmental pH.
Another reason for the rapid development of blue-green algae is an increase in biogenic elements (carbon, nitrogen, phosphorus) in the water. Algae absorb carbon both from organic compounds and from carbon dioxide. Of interest is their ability to photosynthetically absorb carbon in the form of a bicarbonate ion, which is an adaptive property for development in an alkaline environment. The increased content of nitrogen compounds in the water occurs mainly during the decomposition of a large amount of organic matter.
However, the development of algae is most promoted by an increased content of phosphorus entering the water either with decaying organic matter or with mineral fertilizer for plants. In some cases, a population “explosion” of blue-green algae is also observed in the presence of humic acids. Obviously, the reason is that humic acids act as a yellow light filter, and yellow light favors the development of these algae. An increase in water temperature by a few degrees also contributes to the mass development of blue-green algae. This is explained by the activation of metabolic processes and rapid cell division (compared to other plants).
By and large, blue-green algae are evidence of disturbances in the biological system. They easily appear in newly set-up aquariums with an unstable environment. Water changes, shading the aquarium, and improving filtration do not always bring the desired result. Over time, when the ecosystem in the aquarium balances out, i.e., a colony of bacteria capable of processing all organic pollution formed in large quantities in a young aquarium forms in the substrate and filter, and thanks to plants starting to grow well, Cyanobacteria will disappear.
In aquariums with well-established plants, you will never see a growth of blue-green algae. In old aquariums, blue-green algae almost always grow only after sudden changes, for example, when lighting changes, fertilizer overdosing, or the use of any chemical preparation. It is assumed that quite often blue-green algae indicate poor water quality. The reasons for this may lie in: low oxygen content, overdosing of nutrients (including nitrates, phosphates), alkaline pH, excess organic matter, or too frequent feeding of fish, as well as (almost always) too rare water changes.
Methods of combating blue-green algae.
Preventive measures to prevent algae reproduction. 
