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A pond with fish and/or decorative aquatic plants is the dream of many garden owners. Thanks to numerous tips on the Internet and a large selection of material in the hardware store, this dream is quite easy to achieve. But if the water becomes cloudy and green after a while, the garden pond quickly loses its appeal and attractiveness. However, you can do something about it and ensure clear water in the future with very simple means.

green water

Why is the water in my garden pond green?

The formation of floating algae or a large amount of duckweed is usually responsible for the green coloring of the pond water. Cloudy water, on the other hand, is also caused by digested sludge. Since floating algae can multiply very quickly, you should react as soon as possible if you notice them. Incidentally, the flatter and larger a pond is, the more prone it is to the formation of algae. In a small but deep pond, significantly fewer algae form with the same amount of water.

Common reasons for algae formation:

  • too much sun exposure
  • too many nutrients due to over-fertilization
  • leftover fish food
  • too little oxygen
Green water in the garden pond


What can I do against too much sun?

Of course you cannot influence the weather and you cannot push clouds in front of the sun, but there are various ways in which you can shade your garden pond. A parasol that you set up in a suitable place when the sun is strong will certainly help you the fastest. However, this parasol is not a permanent solution, because it would have to be moved several times a day so that your garden pond is really in the shade for a long enough time.

A privacy screen in a suitable place would provide sufficient shade, but it may also obscure your unrestricted view of the pond and thus your enjoyment of it. The solution could lie in a new border planting of your pond. A slightly taller bamboo, for example, provides shade but does not produce as much foliage as a correspondingly large bush or tree. You should definitely build a rhizome barrier around the bamboo, otherwise its long and strong root shoots, the rhizomes, could damage the pond liner after a while and cause a hole.

Muddy water

Why is the water in my new garden pond cloudy?

If you have only recently created your garden pond, cloudy pond water is not a cause for concern. It takes some time until all suspended particles, sand and pond soil have settled. Then you should have a clear view down to the bottom of the pond. However, if the pond water remains permanently cloudy, then you should definitely react.

You may have filled the pond with rainwater because that is a cheap and therefore quite popular solution. However, many nutrients are dissolved in rainwater, which then cause algae to form relatively quickly. Well water is also unsuitable for pond filling, because it is usually low in oxygen. In addition, well water may contain iron and/or manganese. This is especially true for deep wells, the water from shallow or shaft wells is usually more suitable for pond filling.

Garden pond with fountain

water exchange

Can I replace green or cloudy pond water?

At first glance, replacing the cloudy pond water appears to be an attractive and simple solution. Unfortunately, appearances are deceptive. Changing the water is more likely to cause damage than benefit your garden pond. Because with the fresh water you bring additional nutrients into the pond. This promotes the formation of algae instead of inhibiting it.

water purification

How do I get the water clear again?

The first measure against green pond water is to fish away any algae and/or duckweed. You can use a net or a plastic leaf rake for this. This is also suitable for fishing leaves and other unwanted things from the water surface. It is also helpful to use a UVC clarifier, which removes floating algae. You should only use an anti-algae agent if neither children nor animals drink from the pond water or play with it. It inhibits the photosynthesis of the algae and lets them "starve".

biological balance

Care for the biological balance in the long term

In the medium and long term, you should definitely restore the biological balance of the pond and increase the water quality. This is the only way to prevent the pond water from becoming green or cloudy again and again. With a water analysis set you can check the most important water values. For example, the water hardness, the carbonate hardness, the pH value, the nitrate and nitrite content are important.

These values influence each other. If there is an imbalance, you can counteract it with appropriate means and restore balance. It is best to measure the values in spring or summer during the growth phase of your pond plants. During the vegetation break, the values naturally often deviate from the norm.

Optimum water values:

  • pH value (acid-base value): 7 to 8
  • KH value (carbonate hardness): 5 to 12
  • GH value (water hardness): 8 to 12
  • Nitrate: < 0.5 mg per liter
  • Nitrite: < 0.15 mg per liter
Water inlet of a garden pond

UVC clarifier

How does a UVC clarifier work?

A UVC clarifier, also known as a pond clarifier, can be placed in front of a pond pump. The pond water sucked in by the pump then also runs through the primary clarifier. UV light causes the existing floating algae to clump together so that they can be filtered out. In addition, various organisms and germs are to be killed by the irradiation, such as moulds, viruses and parameciums. To ensure that the primary clarifier always works properly and as desired, you should check the UV lamp regularly and clean it thoroughly if necessary. The lamp lasts about a year before it needs to be replaced.


How can I prevent green water in the future?

If you want to prevent green and cloudy pond water in the long term, if you have already improved the water quality and used a UVC clarifier, but still have not achieved the desired result, then you should take further measures.

Measures against green and cloudy water:

  • Plant oxygen-forming plants
  • Install filter system
  • save fish feed
  • Reduce leaf entry
  • prevent sludge
  • Avoid over-fertilization

Be sure to increase the oxygen content in your pond water. So-called oxygen plants excellently prevent the formation of algae and thus green water. Green pennywort or hornwort, for example, is particularly suitable. A filter system also increases the oxygen content in the pond water. If you don't already have a filter, now is the time to get one. You should also replace a filter system that is too small with a larger one. Alternatively, you can install a fountain. However, it will only be able to provide sufficient oxygen in a small garden pond.

With a normal stock of fish, you don't actually have to feed your fish at all. They find enough food in the pond water. If you still want to feed them extra, for example to be able to observe fish, then only give small amounts of fish food. After feeding, no more food should be swimming in the garden pond. Also make sure that there are not too many fish in your pond and also take natural reproduction into account. Because the excrement of your fish also acts as a fertilizer and can cause sludge.

Plants in the garden pond

plant care

Proper care for plants in and around the pond

Fertilize your pond and bank plants only slightly to moderately and never more than is recommended for the individual plants. If your pond water tends to turn green again and again, then you should completely refrain from adding fertilizer for a while. Your pond plants will certainly not be damaged by this. Do not start fertilizing again until you notice that your plants are struggling, not growing enough or not flowering.

If your garden pond is near trees, you may stretch a net over the surface of the water in autumn. This will prevent too many leaves from falling in. If the amount of leaves is kept within limits, then it is sufficient to remove them weekly, thereby reducing the formation of sludge at the same time.

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