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Is Cyanobacteria Harmful to Fish? What You Need to Know? 



Is cyanobacteria harmful to fish?

Cyanobacteria, commonly known as blue-green algae, are aquatic microorganisms that play an essential role in the ecosystem by producing oxygen and serving as a food source for many aquatic organisms.

However, under certain conditions, cyanobacteria can rapidly multiply and form harmful algal blooms, which can pose a threat to the health of aquatic life, including fish.

In this article, we will explore the effects of cyanobacteria on fish and the factors that contribute to the formation of cyanobacteria. We will also discuss strategies for preventing and mitigating the harmful impacts of cyanobacteria on fish and their habitats.  

What is Cyanobacteria? or Blue-green Algae

What is Cyanobacteria? or Blue-green Algae
Cylindrospermum – Blue Green Algae, microscope photograph – Wikipedia

Cyanobacteria, also known as blue-green algae, are a type of bacteria that are found in a variety of aquatic environments, including freshwater and saltwater.

The reason they are referred to as “blue-green” is due to the presence of pigments that give them a color that is a combination of blue and green.

These bacteria are an essential part of the aquatic ecosystem, providing a source of food for other organisms and producing oxygen through photosynthesis. Under certain conditions, some species of Cyanobacteria can form harmful algal blooms (HABs) that produce toxins harmful to humans and animals.

Although cyanobacteria in aquariums typically doesn’t harm fish, it can harm plants by covering their leaves and preventing them from photosynthesizing light, potentially resulting in their death.

This is usually not good for the entire aquarium ecosystem.

How to Identify Cyanobacteria in Aquarium? 

It is extremely easy to identify Cyanobacteria. You can spot it as soon as it begins to spread in your aquarium unless it’s somewhere not visible. 

Cyanobacteria can be identified by their distinctive blue-green color, which comes from the pigments they use to capture light energy. Apart from the blue-green color, these bacteria can appear brown, red, and black. 

Moreover, the smell these bacteria give out when in large quantities is not pleasant.

It has a damp and swampy smell, that can be unbearable sometimes. Most of the time, the reason for the foul smell of your aquarium might be because of the accumulation of Cyanobacteria. 

Therefore, identifying Cyanobacteria in an aquarium is a piece of cake. The odor and the blue-green appearance are the only things needed to identify them. 

Causes of Overgrowth of Cyanobacteria in Aquarium

Causes of Overgrowth of Cyanobacteria in Aquarium
  • Poor Water Quality – One of the primary causes of cyanobacteria overgrowth is poor water quality. Inadequate filtration, overfeeding, and infrequent water changes can lead to the accumulation of organic matter in the aquarium, providing a food source for cyanobacteria. It can be the presence of dead plants, excess food, and so on. The organic matter will lead to high levels of nitrates and phosphates in the water can promote the growth of cyanobacteria. 
  • Excess lighting in the Aquarium – In an aquarium, anything excess is not ideal at all. Therefore, excess lighting contributes to the overgrowth of cyanobacteria. Excess amounts of lighting can inhibit the growth of Cyanobacteria or any other kind of unnecessary stuff. 
  • Inadequate water flow – Poor water flow can lead to stagnant areas in the aquarium, allowing cyanobacteria to settle and multiply. 
  • Introduced in the aquarium – Sometimes, introducing new fish or plants to the aquarium without proper quarantine procedures can introduce new strains of cyanobacteria to your aquarium. 

Is Cyanobacteria harmful to fish? 

Is Cyanobacteria harmful to fish

While the bloom of Cyanobacteria can seem to be a major problem but these bacteria do not harm fish too much. Although, these bacteria can lower the oxygen levels in your tank only if they become accumulated excessively. 

If you have the fear that your fish might consume these bacteria, you need not worry. Fishes are smart enough to differentiate between what they should eat and what they should not.

For if these bacteria are consumed by fish, it will harm them. 

As fish do not consume Cyanobacteria, this means that a clean-up crew will not get rid of these bacteria for you.

You must remove these bacteria from your tank by yourself.

How to get rid of Cyanobacteria (blue-green algae) in fish tank?

How to get rid of Cyanobacteria (blue-green algae) in fish tank?

Having Cyanobacteria is not good in an aquarium, whether it be in small amounts or large amounts they should be removed from your tank. There are lot of ways by which you can remove them from your tank. All of these ways are simple and easy. 

Clean your Aquarium

If your aquarium is experiencing cyanobacteria bloom, then the first step will be for you to clean your aquarium. Therefore, you must get rid of Cyanobacteria on surfaces like rocks, glass, and the substrate. So clean the glass, scrub the rocks, and vacuum the substrate. 

Perform a water change and lower the light intensity for a few days. Then again perform a water change of about 15%. You should do this until all the cyanobacteria go away. 

Improve Water Quality 

The first step in getting rid of cyanobacteria is to improve water quality. This can be achieved through regular water changes, adequate filtration, and avoiding overfeeding.

Removing any decaying plant matter or dead fish can also help reduce the nutrient levels in the water and prevent the growth of cyanobacteria.

Maintaining light and water flow 

Regulating lighting and water flow can also help inhibit the growth of cyanobacteria. Maintaining lighting intensity and duration can highly reduce the growth of cyanobacteria.

Additionally, increasing water flow can help prevent stagnant areas in the aquarium, reducing the likelihood of cyanobacteria settling and multiplying. 

Chemical methods using Erythromycin

Chemical treatments can also be used to get rid of cyanobacteria but should be used as a last resort.

Antibiotic Erythromycin can be effective in treating cyanobacteria, but can also harm beneficial bacteria and other aquatic life.

An adequate amount is 200 mg of erythromycin per 10 gallons of water.

It’s important to follow the manufacturer’s instructions carefully and monitor the water quality closely when using chemical treatments.


Cyanobacteria in a tank is extremely unnecessary and it should be removed at all costs. They give an extremely foul smell and make your aquarium look hideous. But you shouldn’t beat yourself if cyanobacteria accumulate in your aquarium, sometimes something can go wrong and who knows when. Therefore you should get to removing those bacteria from your aquarium. Getting rid of them is easy, you only need to clean your aquarium and maintain the light and water flow.

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