Blue-green algae in the pond

Combat Blue-Green Algae in Your Pond

In the summer months, you may have heard in the news that bodies of water were closed for swimming because they were infested with blue-green algae. Blue-green algae are not actually algae but bacteria. They can also infest your garden pond and create slimy deposits. They are barely noticeable at first, but they can multiply rapidly under favorable conditions. Blue-green algae are significantly more dangerous than relatively harmless green algae. Since they produce toxic sulfur and nitrogen compounds, it is important to act quickly and combat blue-green algae in the garden pond.

What are blue-green algae?

Blue-green algae lack a cell nucleus and are therefore not true algae; they belong biologically to bacteria. Due to their blue-green color, they are called cyanobacteria. Just like true algae, blue-green algae have chlorophyll, which absorbs carbon dioxide and is responsible for photosynthesis. Photosynthesis produces oxygen as a byproduct, but blue-green algae are still harmful and even dangerous. Chlorophyll is overlaid with blue and red pigments responsible for their coloration.

Different types of blue-green algae

Depending on their lifestyle, various types of blue-green algae are distinguished:

  • Some types perform photosynthesis and enrich the water with oxygen as a byproduct.
  • Other types utilize hydrogen sulfide as a reducing agent, release toxic sulfur compounds into the water, and form pond sludge under air exclusion.
  • Additional types use nitrogen and reduce it to ammonia and ammonium, which are toxic to fish and other pond inhabitants even at low concentrations.

What makes blue-green algae so dangerous?

The production of toxic sulfur compounds or ammonia and ammonium makes blue-green algae dangerous for the garden pond. Blue-green algae naturally occur in low concentrations in any water body, but their natural presence is not hazardous. They become dangerous only when they multiply explosively. It is the metabolic products of blue-green algae that occur in high concentrations and are harmful to animals and humans. Inhalation of aerosols containing the metabolic products of blue-green algae can lead to respiratory infections. Ingesting contaminated water can result in diarrhea and vomiting. Skin contact can cause rashes and allergies. Blue-green algae poisoning can lead to the death of animals.

How do blue-green algae enter the pond?

Blue-green algae occur naturally in low concentrations in any water body, including garden ponds, and are not a threat at this stage. They only become a danger when they proliferate extensively. Since blue-green algae are more likely to appear in older and polluted ponds, it is essential to regularly clean your pond and remove impurities daily during the summer months. It’s also crucial to monitor the water quality daily in the summer. If blue-green algae proliferate significantly, they disrupt the pond’s biological balance.

Why do blue-green algae multiply explosively?

Several conditions must be met for blue-green algae to multiply explosively in your garden pond:

  • Excess nutrients such as ammonium, phosphate, and nitrate
  • An excessive fish population
  • Warm, sunny weather and rising water temperatures

In calm and stagnant waters, blue-green algae can proliferate more significantly than in flowing waters. This explains why blue-green algae can appear in garden ponds in high concentrations during the summer. An excessive fish population results in a high nutrient input due to fish excreta and provides an excellent foundation for blue-green algae multiplication. Excessive fish food that remains uneaten also promotes blue-green algae proliferation. Various pollutants like pollen or fallen leaves in the pond can increase nutrient concentration and encourage blue-green algae’s multiplication.

Recognizing blue-green algae in the pond

Blue-green algae are sometimes referred to as slime algae because they can form slimy deposits in the pond. They are not initially recognizable. Only when they have multiplied extensively can they be identified. They form slimy, greasy deposits on the pond walls and on aquatic plants. When blue-green algae have multiplied significantly, these deposits detach and float as dense blue swaths in the water. These brownish-black or metallic blue-green carpets often accumulate at the pond’s edge and in waterways. In rare cases, blue-green algae can also form so-called “pond plums.” They clump together into gelatinous spheres.

Planktonic blue-green algae appear individually or in cell colonies in floating layers. They turn the water into an opaque blue-green brew. When blue-green algae have proliferated significantly, you will notice a foul odor of rotten eggs in the pond water. This odor is caused by ammonia. The pond water becomes imbalanced and leads to the death of the living organisms within. Blue-green algae can be easily distinguished from filamentous algae. They do not form threads but create dense mats. They cannot be removed by brushing them with an algae brush or agitating the water with a stick.

How to prevent blue-green algae

By creating a balanced pond environment, you can do a lot to prevent blue-green algae. When keeping fish in your pond, it is essential to avoid excessive fish populations. Pay attention to the recommendations for the number of fish per cubic meter of water, depending on the fish species. Use high-quality fish food and follow the feeding recommendations to ensure that the food is completely consumed by the fish. Remove fallen leaves and other impurities from the pond daily during the summer months. Provide shade to your pond to prevent excessive sunlight and temperature increase. Monitor water quality daily and change part of the water if the water quality deteriorates significantly. Replenish water when evaporation occurs. Remove filamentous algae regularly, but also algae mats that could be blue-green algae.

Combating blue-green algae

If blue-green algae have multiplied significantly, immediate action is required. Cyanobacteria not only cause aesthetic issues, but their excretions can lead to the death of fish and severe poisoning in humans. Thoroughly remove all blue-green algal mats. Remove fish and other pond inhabitants from the pond and place them in clean water. Relocate aquatic plants, placing them in a container with water. Add an algaecide specifically designed to combat blue-green algae to the pond. Use a pond sludge vacuum to remove pond sludge and pump out the water thoroughly. Clean the pond walls thoroughly and wash off all deposits using various pond accessories. Once you have thoroughly cleaned the pond, refill it with water.

Water plants against blue-green algae

Various water plants not only enhance the visual appeal of your pond but also ensure clean pond water and counteract the formation of algae. They also contribute to preventing the excessive proliferation of blue-green algae. It’s essential to have a healthy mix of plants, and your choices may depend on your pond’s size. Suitable options include:

  • Duckweed
  • Bulrush
  • Elodea
  • Water forget-me-not
  • Water iris
  • Sedges

Various algae-eating fish like roach, minnow, or rudd can help maintain clean pond water and can counteract the excessive multiplication of blue-green algae.


Blue-green algae are not true algae but cyanobacteria. There are various types of blue-green algae that can poison the pond water with their excretions. A natural concentration of blue-green algae is present in every body of water. They become harmful only when they proliferate explosively, typically during warm, sunny weather with high nutrient levels. In such cases, immediate action is needed to avoid endangering the fish and other pond inhabitants. You can prevent blue-green algae by maintaining a balanced pond environment, including a variety of aquatic plants and a reasonable fish population.

Florian Egert

I am Florian Egert, the owner of I live with my wife and two children in Germany in a small village in the countryside.

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