Do you have a mysterious, blue-green slime taking over your aquarium? Or is there a strange smell coming from your fish tank and you can’t find the source? You might be dealing with an outbreak of blue-green algae. In this article, we discuss the causes of blue-green algae and how to get rid of it once and for all.
Blue-green algae (BGA) is not actually an algae but rather a cyanobacteria – a very diverse and resilient group of bacteria that uses photosynthesis like plants. In freshwater aquariums, it’s known for its vivid blue-green color, but it can also appear in shades of brown, black, or even red. You may see it start off as a little spot of green algae that eventually grows into a thick slime stretched over your gravel, decorations, and plants. While cyanobacteria in aquariums does not usually harm fish, it can potentially kill your plants if their leaves are covered and can no longer photosynthesize light.
Another way to identify blue-green algae is by its distinct odor. People have described the smell as earthy, musty, swampy, and foul. Once you have learned to recognize the scent, it’s possible to detect cyanobacteria up to two weeks before it’s even visible in the fish tank.
Blue-green algae is actually a type of photosynthetic bacteria that comes in blue, green, brown, black, and red colors.
Many studies have been done to figure out what causes cyanobacteria blooms, since they can have a deadly impact on the environment. While there are no definitive answers yet, they commonly occur in warm, slow-moving, and nutrient-rich bodies of water. In the aquarium hobby, we have frequently seen blue-green algae pop up wherever organic waste has a chance to stagnate in certain areas of a fish tank. This can happen if:
Based on these possible causes, the first step is to manually remove as much of the slime as possible using a siphon, toothbrush, or algae scraper. (Your clean-up crew won’t be of much help, since animals don’t like the taste of blue-green algae.) Remove any excess nutrients by doing water changes more frequently, cleaning the filter regularly, and reducing the amount of fish or food going into the aquarium (if overfeeding is a problem). Improve the water flow by using a stronger filter, adding a powerhead, or moving decorations and equipment around in the tank.
Cyanobacteria uses photosynthesis to create energy, so some people suggest turning off the aquarium lighting for three to seven days to starve out the colony. However, this method can end up harming your plants (which also use photosynthesis) or causing spats among the fish. Plus, the blue-green algae often returns within a few weeks.
Many people have trouble dealing with this stubborn bacteria, but fortunately, it is very weak against a certain antibiotic called erythromycin (also sold as Mardel Maracyn® in the United States). This medicine is safe for fish, plants, and invertebrates, and it will not harm the beneficial bacteria in your aquarium.
To begin treatment, scrub off as much of the blue-green algae as possible and remove it with a siphon. After vacuuming the substrate and refilling the tank, add one full dose of Maracyn (which is 1 packet per 10 gallons of water), and let the aquarium sit for one week before doing another water change. The earlier you treat the outbreak, the easier it is to eradicate. If the blue-green algae is very thick and widespread, you may need to repeat the treatment several times to completely remove the colony.
If you address the underlying causes of cyanobacteria and treat it with Maracyn, you should have no problems getting rid of it in your fish tank. If you’re struggling with another type of algae, check out our full guide on how to fight the top 6 types of algae in freshwater aquariums: