Why Does the Water in the Pond Turn Green?

Why Does the Water in the Pond Turn Green

Green pond water not only affects the appearance of your garden pond, but it worsens the living conditions for fish and other animals, as well as for plants. This green coloration is caused by algae and occurs mainly in the summer months. There is no such thing as completely clear pond water. However, it can quickly tip over if it becomes a green soup. This means the death sentence for your fish. Learn how to get rid of this green coloration.

Green Pond Water in Newly Created Ponds

If you have newly constructed your pond and filled it with water, you will be pleased with the clear water. You hope it will stay that way. However, this is not always the case. If the water was clear just a moment ago, it could become discolored a short time later. This can often be observed in newly created garden ponds.

Why Does the Water in the Pond Turn Green?

In new ponds, the green coloration is because the biological system must first build-up. It is not yet coordinated and must first settle in. Floating algae cause green coloration. They can quickly get out of hand. With photosynthesis, the algae and the plants in the water form the green pigment chlorophyll. Only when there is enough light can the plants photosynthesize. The pigment chlorophyll enables the plants, but unfortunately, the algae, to survive and ensures that the algae multiply quickly.

Nutrients Responsible for Floating Algae

Suspended algae and other algae will colonize if there are too many nutrients in your pond. This can have various causes:

  • lack of pond hygiene
  • leaves falling into the water
  • dead plant parts
  • fish food that sinks to the bottom and is not eaten up
  • too many fish pollute the pond with their excrements
  • too many nutrients can also be in the water if few plants are in your pond. This is often the case during the colder months when the plants are not yet growing profusely. This already creates the basis for contamination

You can counter this problem by always keeping your fish population under control. If the fish has increased, you can give away fish you cannot use to friends or offer them at a swap meet.

You should also use high-quality food. You will need less of it, as the fish will get full of it faster.

You should remove dead plant matter and pond sludge when cleaning the pond. You should also regularly remove leaves that have fallen into the pond with a landing net.

It is best to check the water levels daily and take the appropriate countermeasures to prevent the pond water from tipping over during the summer months. Always ensure good water quality and practice consistent pond hygiene, preventing algae formation.

Animal Helpers in the Fight Against Algae

You do not have to resort to the chemical club to fight algae. Animal helpers are hungry and will help you decimate the algae. Suspended algae are the favorite food of water fleas and hoppers. These little animals are a part of the food chain for various fish. So, you should make sure to have plenty of water fleas and hoppers in your pond. Such small crustaceans indicate a good pond ecology.

Grass carp are also recommended to combat floating algae. They reach a considerable size and are suitable only if your pond is large enough. Alternatively, you can use pond mussels to help you fight algae.

Help With the UV Filter

To prevent the formation of floating algae, you can use a UV filter. This UV clarifier is connected to the pond filter system. Meanwhile, there are intelligent UV clarifiers that do not work constantly. They save energy and counteract the floating algae and the filamentous algae. It would be best if you did not use the UV clarifier permanently. The intelligent clarifier switches off when it is not needed. To effectively combat algae, it depends on the correct positioning of the clarifier in the garden pond.

Reduce Nutrients in the Pond

The formation of algae is promoted by nitrate and phosphate. A high incidence of light also encourages the formation of algae, as more chlorophyll can then be formed. You can reduce the appearance of such nutrients with various measures.

Light as a Growth Promoter

Plants and also animals need light. Too much of it is harmful, as it is also a significant growth factor for algae. By shading the pond, for example, with a sunshade or a sun sail, you can reduce the growth factor of light, especially on hot summer days. If you want to create a new garden pond, opt for a semi-shaded location.

Fill Pond With tap Water

If you use well or cistern water to fill your pond for the first time, you are already creating a good foundation for algae to grow. You can avoid this by using tap water, which contains fewer nutrients.

Use the Right Filter

You can reduce the formation of nitrate with the right filter. The biological filter converts nitrogen, excreted by fish as ammonia, into toxic nitrite and non-toxic nitrate. Nitrifying bacteria live in the biological filter and produce nitrate. You can remove mud, fish feces, and leaves from the pond with a mechanical filter.

Pond Cleaning at Least Once a Year

At least once a year, you should give your pond a beauty treatment and clean it thoroughly. A thorough cleaning can be done in the fall when you winterize your pond and in the spring when you get the pond ready for the new season. You should check the plants for dead parts, divide overgrown plants and remove mud and other debris with a mud vacuum. It is better to clean twice a year. You will deprive the algae of essential nutrients when you remove the mud.

How to Remove Nitrate From the Pond

To remove nitrate as an essential nutrient base for algae, you have two options.

You can reduce the formation of nitrate with a lush abundance of plants. However, these plants require regular maintenance. If you do not have fish in your pond, because it is a swimming pond, an extensive planting and sump zone is vital so that the pond can regenerate well. This zone should be at least twice the size of the water surface. You can also create a filter trench for cleaning and regeneration.

If there are fish in your pond that need to be fed daily, the planting zone should be even five to ten times as large as the water surface. For growth, these plants need a lot of nitrates. They leave little for the algae.

During the season, the plants have absorbed and stored a lot of nitrates. If you winterize the pond, you must cut back the plants. This way, you prevent large amounts of nitrate from being released back into the water.

If you want to make your work easier or do not have enough space for many plants, you can prevent nitrate formation with denitrifying bacteria. Such bacteria are offered in the form of tabs. The bacteria metabolize nitrate to gaseous nitrogen, which is released into the air.

Depending on the nitrate level, it may take several weeks for the nitrate to gasify. Once there is no more nitrate, it takes about two more weeks for the algae to die. It can take about three to eight weeks for the water to clear with these bacteria. The water will remain clear if you add the bacteria every two weeks.


Suspended algae are responsible for the green coloration of the pond water. They occur in newly created ponds because the biological system is not yet in place. Suspended algae also happen when there are too many nutrients in your pond. Flower pollen leaves that have fallen into the water, dead plant matter, fish food, and fish excretions lead to high nutrient levels and encourage algae formation. You should pay attention to good pond hygiene and clean the pond preferably twice a year to avoid this.

Florian Egert

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

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