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How to Get Rid of Blue-Green Algae in Planted Aquariums: Expert Tips

Lindsey Stanton Profile Picture

By Lindsey Stanton


Have you noticed an iridescent, blue-green sheen on the surface of your tank’s water or on your décor? This may be blue-green algae, which can be difficult to get rid of. Unlike some types of algae, blue-green algae can cause illness if accidentally ingested by humans or pets. It also stinks, causing your aquarium to smell fishy and stagnant.

So, what exactly is blue-green algae and how do you get rid of it? Keep reading for more information!

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What is Blue-Green Algae?

Blue-green algae is actually a misnomer because it isn’t algae at all. Blue-green algae is an outdated term for a type of cyanobacteria, which is a group of bacteria that thrives in wet, warm environments with high lighting. It prefers an anaerobic environment with high levels of excess nutrients like phosphate.

If you accidentally consume blue-green algae or get it in your eyes, it can lead to nausea, vomiting, headaches, sore throat, coughing, and eye itchiness or pain. Pets that may drink out of your aquarium can have similar symptoms. Some types of cyanobacteria can be deadly to fish if consumed, as well as by causing oxygen levels in the tank to drop. It can also convert nitrogen into ammonia, causing ammonia levels to rise.

Blue-green algae is not a sign of a poorly kept tank. Excess phosphates can get into your tank via tap water and cyanobacteria can hitch a ride into your tank on plants and items like rocks.


green cyanobacteria in aquarium tank blue green algae
Image Credit: Vojce, Shutterstock
How to Get Rid of Blue-Green Algae:
  • Reduce Tank Lighting: Blue-green algae is photosynthetic, which means it gets energy for growth and reproduction from light. To get rid of blue-green algae, you may need to reduce your tank lighting until you are able to get it under control.
  • Reduce Nutrients: Overfeeding your tank and even tap water can lead to excess phosphates and nitrogen, which blue-green algae will feed off of. Reducing the number of feedings per day or the amount of food per feeding will help to begin starving out the blue-green algae. You can purchase test kits to see if your tank has high phosphates. Testing your tap water to get a baseline can help you keep track of this as well.
  • Clean the Tank: To remove blue-green algae, you will need to scrub tank décor and glass. You also may need to remove your substrate and clean it thoroughly. Physically removing blue-green algae from surfaces within your tank will make it easier to get out of the water and get under control.
  • Water Changes: Physically removing blue-green algae via water changes will help you get it under control. When it comes to water changes, though, you should only remove around 20-25% of the tank water at a time. Removing too much water may shock your fish or even introduce more nutrients into the water, allowing the blue-green algae to come back just as strong.
  • Use Phosphate Removal Treatments: There are powder and liquid treatments that can be added to your tank water to help decrease or remove phosphates in the water. You can also use special filter cartridges or other filter media that will draw out phosphates as the water processes through.
  • Optional Treatment: If you have tried all of the above options and are still struggling with blue-green algae in your tank, then you can use the medication Erythromycin to eliminate the bacteria. This medication is available online and in pet and fish stores in special formulations that are intended for aquarium use. This treatment option should only be used if all other options have failed, though. Since Erythromycin is an antibiotic, it will kill the cyanobacteria, but it will also kill beneficial bacteria within your tank as well, which may crash your cycle and cause ammonia and nitrite levels to rise.
Image Credit: Choksawatdikorn, Shutterstock
What Won’t Get Rid of Blue-Green Algae:
  • Removal Only: Cleaning your tank, décor, and substrate is an effective part of getting rid of blue-green algae, but just cleaning will not eliminate the other factors allowing the growth of blue-green algae, nor will cleaning successfully remove all traces of blue-green bacteria from your tank.
  • Algae Eaters: Algae eaters, like Plecostomus and Siamese Algae Eaters, can be beneficial to keeping algae populations in your tank under control. Unfortunately, they will not eat blue-green algae.
  • Water Changes Only: Just like with cleaning the tank, water changes alone are not enough to thoroughly remove blue-green algae from your tank. Blue-green algae can hide anywhere within your tank and will grow on surfaces and in open or clear filters, so just changing the water won’t remove enough blue-green bacteria to control the levels in your tank.
Image By: Chokniti Khongchum, Shutterstock
Prevention of Blue-Green Algae:
  • Avoid High Light: Don’t use higher light than is absolutely necessary for your plants and animals in your tank. High lighting levels can lead to cyanobacteria blooms and allows for the growth of multiple types of algae as well. Using the minimal amount of light to maintain your tank will help reduce the risk of blue-green algae growth.
  • Routine Cleaning: Cleaning your tank glass, substrate, and décor on a regular basis will help prevent the potential for the buildup of blue-green algae on surfaces. Cleaning the substrate will pick up rotting food and waste, taking away a source of nutrients for blue-green algae.
  • Treat Incoming Plants: Treating new plants with a bleach dip of 1:20 bleach to water ratio will kill blue-green algae that may be lurking on the plants. It will also kill most parasites and pests that might hitchhike into your aquarium.
  • Avoid Overfeeding: The easiest way to prevent blue-green algae is to avoid overfeeding your tank. Most fish only need to be fed once or twice daily, if even that much. Shrimp and snail tanks can go longer without food since they will snack on biofilm and goodies growing in the tank. There are a few rules of thumb that pertain to how much you feed your fish, whether it’s limiting it to what they can eat in 2-5 minutes or only feeding an amount of food equal to the size of your fish’s eye, but none of these are exact. Keep an eye on how much food is being consumed at meal times in your tank and how much is falling to the substrate and not being picked up. Use this as a guide to increase or decrease your feeding amount.
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In Conclusion

Many people find that once they have blue-green algae, they never fully get rid of it. This makes prevention extremely important, whether you’re preventing blue-green algae from getting into your tank or preventing a blue-green algae bloom. Prevention is easy but does take some vigilance on your part.

Blue-green algae can be dangerous, both to you and your aquarium. It needs to be dealt with as soon as you spot it to reduce the risk of fish loss and illness. Blue-green algae can be difficult to eliminate and control, but it is possible with the right combination of treatments!

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Featured Image Credit: mivod, Shutterstock

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Authored by

Lindsey discovered her passion for fish keeping after a junior high school field trip to the Shedd Aquarium in Chicago. Prior to becoming Editor-in-Chief of It's a Fish Thing, Lindsey studied marine biology at the University of California-Santa Cruz. She loves goldfish, tetras, and mystery snails, and recently began experimenting with a saltwater aquarium.

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