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4 Ways to get rid of Blue-Green Algae in Aquarium

Algae is every serious aquarium hobbyist’s worst nightmare. This pernicious, fast spreading, non-flowering aquatic plant can very quickly overrun a well-kept aquarium and reduce all your hours of devotion and hard work to nothing.

That is the reason why aquarium owners will go to such great lengths to counter this threat, including treating water, changing the environment or installing specialist equipment.

Algae is one of the most destructive forces to be found in any home aquarium.

Not only does this green-blue gunk ruin the appearance of your tank, but it can also do extreme damage to the ecosystem. This can use up precious resources in your tank, throw the water chemistry out of balance, and inevitably damage the health and well-being of your fish and other aquatic creatures.

So, if you want to keep your tank in tip top shape, and learn how to get rid of blue-green algae in aquarium, then you have come to the right place.

In an effort to keep your tank as healthy and aesthetically pleasing as possible, we’ll give you all the hints and tricks to prevent algae taking over, and guide you in tackling this menace if it does crop up.

How to get rid of Blue-Green Algae in Aquarium

Before you tackle the daunting task of getting rid of blue-green algae from your precious aquarium, you should understand the fundamentals of what this attacker is, and where it comes from.

Algae is an aquatic plant that consists of very simple cellular organisms. It breeds quickly in the right conditions, usually living on the surface of water bodies and often spreading over multiple square meters per day.

Like most aquatic organisms it requires oxygen to survive and also needs a healthy supply of sunlight.

Algae can come in a number of different forms, from dense mats to very fibrous, filament-like material. It can survive in most regions of the world, but generally does better in hotter climates.

The type of algae that we will focus on in this article is the blue-green algae. This is one of the most widespread algae types that bother home aquariums.


Blue-green algae cultured in specific media by Joydeep (CC BY-SA 3.0)

Blue-green algae is bit different from your standard green stringer algae. The most significant difference is that unlike other algae, it is not actually a plant. Instead it is a form of rapidly growing bacteria.

The correct scientific name for this bacteria is cyanobacteria, but most experienced hobbyists will know it best as blue-green algae. This is a photosynthetic bacteria that dwells and colonizes happily in soils and substrates all over the world.

This is actually a phylum, which means that there are a host of different species within this group. Some of this species are extremely simple unicellular organisms, while others are more complex filament-like species.

In any case the end result is the same, this algae will quickly grow, consume resources in your tank, and destroy the peaceful equilibrium that existed before. But only if you do not learn how to get rid of blue-green algae in aquarium.

Dangers of Blue-Green Algae

There are a number of dangers with this blue-green algae. First of all, it is not an attractive sight in a tank, and this can be devastating for any person who has put a lot of energy into crafting the perfect aquarium.

Usually, if there is enough of the blue-green algae that accumulates in the substrate of your tank, this will tend to float to the surface and form a very unhealthy-looking scum.

Generally, this will be a greenish-blue in color, but that may vary. It may also be greenish, brown or even red. One of the most destructive effects of algae is that it depletes the oxygen levels in the tank.

Dissolved oxygen (DO) is absolutely necessary for fish and other aquatic creatures to survive, so when the level of DO drops below a certain level your fish will not be able to survive. This can be a quick process unfortunately.

Finally, some varieties of cyanobacteria may be toxic to some kinds of aquarium fish. You may notice your fish having illnesses of disease due to them innocently eating the harmful blue-green algae.

4 Ways to get rid of Blue-Green Algae in Aquarium

Video: “How to Kill Cyanobacteria – Natural vs. Chemical?”

There are a number of steps you can take to get rid of blue-green algae in aquarium, as will be detailed here.

1. Manual Removal

Manual Removal is definitely one of the simplest possible solutions, but it can also be one of the most time consuming.

It is probably a good step to take at the start, but be aware that you will probably not be able to physically remove all traces of the bacteria by hand, and hence it can quickly grow back. This is especially true if you have not dealt with the underlying issues for the algae bloom.

To start a thorough manual removal of blue-green algae there are a number of things you should do. Start by fishing out the scum from the top of the tank and removing the layer at the bottom of the tank.

The bacteria will usually stick together in sheets, making it slightly easier to remove. Next you should carefully scrub and scrape the glass of your tank. Next remove any rocks from the bottom of your tank and thoroughly clean the floor and substrate. Use of a vacuum to clean the substrate is highly recommended.

A thorough physical cleaning should be the first step you take before you progress to using any more drastic measures, and regular thorough cleaning should be a normal part of your tank maintenance routine anyways.

Be aware that any piece of algae that escape your attention can become the breeding ground for new colonies which can very quickly grow to the same size as the previous problem.

You may have to throw entire plants or ornate decorations away if you cannot clean them thoroughly enough, or they are just too damaged or infected by the algae bloom.

2. Light Detox

Light Detox is another possible solution. This is quite extreme but is usually effective at dealing with troublesome outbursts of algae bloom.

The reason for its effectiveness is that algae thrive in sunlight or artificially lit conditions, and they cannot survive for long without that nurturing light. On the other hand, most of your carefully tended aquatic plants and fish will be far more resilient. Hence you can try the following treatment.

Completely cover your tank with some sort of blackout material to remove the light from the area. This treatment will only work if absolutely all ambient light is excluded from the tank.

You will need to keep the light out for a number of days. Be sure to feed your fish before you start the treatment, because you should minimize all sight or contact with your fish while the light detox is ongoing. You can quickly peek at them after a few days to make sure they are okay, but most breeds of tank fish are hardy and resilient enough to be left alone for a while.

After a week perform a large capacity water change and restart the treatment, again keeping the tank completely in the dark. Again, feed your fish sparingly if at all, as any uneaten food will help the algae to survive.

Finally, after approximately two weeks in the dark, you should be able to remove the covers, do yet another water change, and your tank should mostly be returned to normal. You can remove the remains of the dead, light-starved algae by hand, if required.

This is a drastic enough measure that should only be attempted after manual removal and cleaning has failed.

3. Water changes and Adequate Filtration

Water changes and adequate filtration are other necessary components of removing blue-green algae from a fish tank.

One of the major reasons for the excessive growth of algae in a confined tank is the buildup of nutrients and fish wastes. Fish produce waste such as ammonia and nitrites when they consume their food, and this cocktail of chemical and substances can be a real attraction for some species of cyanobacteria.

By installing good filtration, the positive bacteria will help to break down these byproducts of fish waste into much more harmless substances, hence helping to keep the level of negative bacteria to a minimum. If the level of nutrients and waste rises to such a level that an algae bloom starts to happen, then you should definitely consider a water change.

Changing the tank water will remove some of the outsider bacteria, but it will also help restore the nutrient balance in the tank to a more normal level. Remember you should always replace water with properly cycled water of the right temperature, and it is best to change about 20% of the tank water at a time. With a dire algae bloom in progress, you might have to replace much larger amounts of tank water, up to 80 or 90 per cent.

4. Chemical Treatment

Chemical treatments are also available to help deal with blue-green algae growth, but it is only recommended that these are used as a last resort. In fact, some of these chemical treatments should only be carried out by very experienced hobbyists or in the presence of an expert.

Chemical treatments can be highly effective, but they also carry a high level of risk. The reason for this is that they can also indiscriminately kill the ‘good’ bacteria in your tank. This is the bacteria that is essential to complete the nitrogen cycle and perform the act of biological filtration, breaking down fish waste and keeping the chemical balance in your tank. If these bacteria colonies are killed then your fish are at risk of dying soon after.

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One chemical treatment is the antibiotic erythromycin, which will target the bacteria in your tank. It should be dosed with great care, and you should carefully monitor the ammonia and nitrite levels in your tanks in the following weeks to make sure the good bacteria have not been wiped out.

Hydrogen peroxide is another extremely effective bacteria killer, but the same risks apply. If you do decide to use hydrogen peroxide to dose your tank, then be sure to take expert advice and carefully follow the instructions on the bottle.

Never apply hydrogen peroxide to the tank water as long as you have fish, shrimp, snail or any other living creature in it!

Blue-Green Algae Prevention

When it comes to dealing with all types of algae, the old saying that ‘prevention is better than cure’ really rings true. Algae blooms will always be a recurring issue if you do not deal with the root causes of this explosive algae growth. Most commonly this means a poor cleaning routine, inadequate filtration or a chemical imbalance.

There are a number of preventative measures you could take.

Firstly, do not overfeed your fish. Overfeeding is one of the leading causes of algae growth, because the algae love the undigested nutrients that settle on the bottom of the tank as a result of not being eaten.

Too much sunlight is another important factor. If you have a recurring issue with algae then consider placing your tank in a more shaded area or installing some sort of artificial cover.

Perform regular, gradual water changes in your tank, to prevent the level of nutrients becoming too high.

Finally, if none of these simple measures work you should consider planting more aquatic plants to compete with the algae for resources, or install a better filtration system. These simple tips and techniques could save you a lot of hassle in the long term.


How to get rid of blue-green algae in aquarium is a very relevant and important topic. If you let this bacteria run wild then it will not take long for it to turn your exquisite aquatic oasis into a post-apocalyptic green water world.

Like most problems, you should foresee this problem and take actions to mitigate it. This includes regular cleaning, water changes and other preventative measures.

However, if you do experience an algae bloom it’s not the end of the world. There are a number of techniques you can use to alleviate the problem, from simple manual removal to more complex chemical treatments.

Often a light detox combined with extensive water changes are the most effective treatment.

Aaron Boyd
Aaron Boyd

Hello, I’m Aaron Boyd, the proud owner and author behind Aqua Movement. I hope my article was able to answer your questions. If you want to learn more about me, click the home icon above.

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