Cyanobacteria or Blue-Green Algae in an Aquarium

Causes, Remedies, and Preventive Measures

Green Cyanobacteria in an aquarium

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Cyanobacteria, formerly called blue-green algae, are not really algae but are a phylum of photosynthetic bacteria that live in moist soils and water. The species' structures can be unicellular to filamentous and some species are colonial. It can grow quickly and can cover the substrate in the aquarium. When disturbed, it comes off in sheets. It is also called slime algae or smear algae, which is an appropriate name, as it is very slimy and often gives off an unpleasant swampy or fishy odor. A severe overgrowth of cyanobacteria can gather into a foamy scum at the surface of the water. It is typically blue-green in color, but it can be greenish-brown to black or even red in color.

Some species can be toxic if ingested by fish or other animals and can kill fish indirectly by causing oxygen levels to drop below the threshold for fish survival. But, Spirulina is an edible and nutritious species of Cyanobacteria.

Cyanobacteria produce oxygen by photosynthesis and can also convert inorganic nitrogen (N2) into ammonia.


Overgrowth of this organism generally occurs when there are high levels of dissolved wastes and nutrients in the water, such as nitrate and phosphate. Phosphate, in particular, is a prime contributor to the growth of cyanobacteria.

The buildup of excess nutrients and dissolved waste may be due to lack of water changes and regular maintenance, overfeeding, or because the tank is new and the beneficial bacterial colonies have not become established. Some city tap water can be high in phosphate, making cyanobacteria more likely to grow, but it can even appear in a well maintained, mature aquarium.

The primary causes of blue-green algae in your aquarium include:

  • Cyanobacteria was introduced into the aquarium (on a plant or from the water source)
  • Excess light
  • High levels of organic wastes
  • Anaerobic conditions


Once established, cyanobacteria are rather difficult to eradicate. One way to eliminate it is by taking steps to reduce the nutrients in the water and mechanically removing the cyanobacteria itself. Start by scraping the glass, scrubbing the rocks and plants, and vacuuming the substrate. Perform a partial water change of 20 percent and turn the lights in the tank off for three days. On the fourth day turn the lights back on and perform another 10 to 15 percent water change. That should get rid of the algae overgrowth and reduce the elevated wastes and nutrients that support its growth. If there is still cyanobacteria or algae growth, the process should be repeated.

Be aware that the cyanobacteria will return if the underlying causes are not corrected permanently. In fact, it may never be truly eliminated. However, regular water changes, maintenance, and preventive measures will eliminate the recurrence of a cyanobacteria overgrowth.


  • Another treatment option is the antibiotic erythromycin, which will kill the cyanobacteria that cause slimy growth. However, the use of erythromycin can also kill beneficial bacteria in the aquarium and should be used with care. If such treatment is used, monitor ammonia and nitrite levels closely for several weeks.

Ways to combat a blue-green algae overgrowth in your fish tank:

  • Reduce light
  • Partial water changes
  • Physical removal
  • Clean tank well
  • Use phosphate removing water treatments or filter pads
  • Add 200 mg erythromycin/10 gallons of water

Unlike brown algae, cyanobacteria are not eaten by plecostomus and other algae-eating fish. So, do not count on these fish to help clean up your blue-green algae problem.


As with any algae, keeping the aquarium clean and performing regular water changes are the best preventative measures. When water changes are not routinely performed, nitrate and phosphate will rise, which encourages algae and bacterial growth of all types. Performing small water changes every week or two will keep these nutrient levels low.

Overfeeding is one of the biggest causes of excess nutrients in the water that increases cyanobacteria and algae growth. Most fish do not need more than one or two feedings a day, and then only feed an amount that will be eaten in three to five minutes. If food is visible on the bottom after 5 minutes, you are overfeeding your fish. Excess light, particularly direct sunlight, is another common cause of cyanobacteria and algae growth. Avoid placing a tank in a location where it will get direct sunlight.

Unfortunately, it is still possible to get cyanobacteria or algae in spite of regular maintenance. In fact, small amounts of algae growth are normal. Prompt attention to sudden algae growth will prevent more serious problems.

Top ways to prevent a cyanobacteria bloom:

  • Avoid overfeeding fish
  • Avoid excess light
  • Perform regular water changes
  • Do regular aquarium cleaning
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