How to Remove Blue-Green Algae From Aquariums

AKA Slime Algae

Cyanobacteria, SEM
Cyanobacteria. Science Photo Library/Steve Schmeissner/Getty Images

Cyanobacteria, often called Blue-Green Algae, grows quickly and will cover all surfaces in the aquarium in short order. When disturbed it comes off in sheets. It is also called Slime 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 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.


Cyanobacteria, Blue-Green Algae, Slime Algae, Smear Algae


  • Cyanobacteria organism introduced to the tank
  • Excess Light
  • High levels of organic wastes
  • Anaerobic conditions

Also referred to as Slime or Smear Algae, Blue-Green Algae is not truly an algae. Instead, it is Cyanobacteria, an organism that lies somewhere between algae and bacteria. Overgrowth of this organism generally occurs when there are high levels of dissolved wastes and nutrients in the water, such as nitrates and phosphates. Phosphates, in particular, are a prime contributor to overgrowth of Blue-Green Algae.

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. However, because Cyanobacteria can fix its own nitrogen, it can appear even in a well maintained matured tank.


  • Reduce light
  • Partial water changes
  • Physical removal
  • Clean tank well
  • 200 mg erythromycin phosphate/10 gallons water

Once established, Blue-Green Algae is rather difficult to eradicate. One means to eliminate it is by taking steps to reduce the nutrients and mechanically removing the algae itself. Start by scraping the glass, scrubbing the rocks and plants, and vacuuming the substrate. Perform a partial water change of 15-20% and turn the lights in the tank off for three days. On the fourth day turn the lights back on and perform another 10-15% 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 Blue-Green Algae growth, the process should be repeated.

Be aware that the algae will soon return if the underlying causes are not corrected permanently. In fact, it can never truly eliminated. However, regular water changes and maintenance will eliminate the re-occurrence of an overgrowth of Blue-Green Algae. Follow the steps listed in the prevention section to ensure re-growth doesn't occur.

Another treatment option is Erythromycin, which will kill he Cyanobacteria that causes the slimy growth.

However, use of Erythromycin can also affect the 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.

Note: Algae eaters do not eat Cyanobacteria.


  • Avoid overfeeding fish
  • Avoid excess light
  • Regular water changes
  • Regular aquarium cleaning

As with any algae, keeping the tank clean and performing regular water changes is one of the best preventative measures. When water changes are not routinely performed, nitrates and phosphates will rise, which encourage algae growth of all types. Performing small water changes every week or two will keep nutrient levels low.

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

Unfortunately, it is still possible to get 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.