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Different types of algae can be found in every pond. They are essential for life, serve to feed microorganisms such as water fleas and are an important supplier of oxygen in the water due to their photosynthesis. Occurring in moderation with a normal nutrient concentration in the pond, there is no need to worry. However, when they occur en masse, it is a sure sign that the balance of the pond is out of whack. Then quick action is called for.

floating algae

Colloquially, this type of algae is also known as green algae designated. They are microscopic, floating freely on the surface of the water as a visible green veil. Especially in spring, when the nutrient supply in the pond is still very high because the aquatic plants and other microorganisms are not yet active, floating algae develop explosively. This process is also known as algal bloom. This is marked by

  • cloudy and green water
  • a visibility depth of only a few centimetres

tip: Normally, a slight turbidity of the water is completely normal, even in natural bodies of water. A visibility depth of about one meter is sufficient.

During the algal bloom, the algae produce organic material or die off and sink to the bottom. There it begins to a layer of sludge to rot, but deprives the water of oxygen. This layer of sludge in turn forms the basis for new algae growth.


However, excessive algae growth is not always a sign of an ecological imbalance in the pond. Algae growth can be observed in newly created ponds or when the water is changed. However, it wears off after a week or two. Normally, low concentrations of floating algae in the pond are not a concern. However, they can multiply quickly under suitable conditions.

  • increase in water temperature
  • Sun exposure in spring and summer
  • high nutrient content of the water
  • increased phosphate content

Phosphate is the basic nutrient for algae. Floating algae therefore find very good conditions for rapid multiplication in ponds with an increased phosphate content. Even a concentration of phosphate above 0.035 milligrams per liter can lead to an algal bloom.

An excessive phosphate content in the water can have various causes:

  • Flushing in nutrients from the environment through rain (lawn fertilizer, nutrient-rich soil)
  • excess fish food and fish excrement at the bottom of the pond
  • dead algae

Why remove algae?

In principle, algae are useful plants in the pond. They extract nutrients from the water and supply oxygen in return. No other plant or technique is normally as effective as floating algae.

  • every second oxygen molecule comes from algae
  • convert three times as much carbon dioxide into oxygen as any other crop

On the other hand, algae extract just as much oxygen from the water at night as they produce during the day. Due to the large number of floating algae in the pond, there are strong oxygen fluctuations between day and night. A deficiency can very quickly become a threat to aquatic plants and fish. Other side effects of excessive algae stock are, for example

  • a low KH value of the water (carbonate hardness) and
  • an elevated pH

It should also be noted that dead algae on the bottom release nutrients with the help of oxygen as they decay. The resulting layer of sludge forms the basis for renewed algae growth. If nothing is done, the pond eventually threatens to silt up over time.

tip: Normally, when there is an algal bloom, patience is required. As a rule, the propagation cycle is interrupted by the algae themselves after two to three weeks.

However, if this is not the case, then action must be taken.

remove algae

Now there are various methods of banishing algae from the garden pond. The specialist trade offers various means. However, it should be borne in mind that in some cases only the symptoms are combated here and not the causes. This clears the water, but algae growth continues. Fast action is necessary, especially when algae appear more frequently. Sometimes pond filters are no longer sufficient here because the algae cannot be absorbed by the filter due to their size. There are other solutions for this:

UV-C water clarifier

A permanent solution for combating algae is the use of a UV-C water clarifier in the pond. However, this UV-C lamp only lasts for one pond season. It should therefore be renewed every spring. The function of this device is relatively simple:

  • Pond pump directs water through UV-C water clarifier
  • then reaches the mechanical filter
  • ultraviolet light destroys the hereditary information of the algae
  • clumping algae
  • Filter transports residues away
  • UV radiation harmless for pond and inhabitants
  • UV-C lamp already integrated in many pond filters
  • if not, easy integration into the water cycle is possible
  • Important Installation in front of the filter, otherwise the algae residues will not be absorbed

Pond sludge sucker and skimmer

Both devices are reliable helpers in pond care.

  • Vacuum removes nutrient-rich deposits from the soil
  • Skimmer removes floating debris
  • Water is sucked in at the pond surface and directed to the filter
Sludge suction at the garden pond

water depth and light conditions

Lots of sunlight accelerates algae growth, so a garden pond should be in the shade for at least a third. The amount and depth of water also have a direct influence on the development of algae. Small, shallow ponds have to struggle with algae problems much more often than larger systems. The use of different plants for shading can be helpful:

  • Plants with large floating leaves (water lily, lotus, sea jug, frog bite)
  • Plants under the water surface with a simultaneous cleaning effect such as water hose and waterweed
  • for the riparian zone water bulrush, loosestrife, cattail, arrowhead, iris
  • When planting, do not put too much nutrient-rich soil in the pond
  • Use of special aquatic plant soil

tip: Check the pond water regularly with a pond test set. The ideal GH value (hardness of the water) should be between 8 and 12 dH.

water movement

It is generally known that constant water movement is not exactly conducive to the formation of algae. It is therefore advisable to think about alternatives such as

  • Installation of fountains or watercourses
  • simply place a stable oak branch in the water
  • thereby lowering the pH value by releasing tannic acid
  • poor living conditions for algae
  • Remove the branch before decomposition, otherwise the process is reversed

pond plants

Aquatic plants are a kind of biological filter. Strong growing plants remove nutrients from the water. The more plants there are in a pond, the less food there is for the algae. They should therefore not be missing in any pond.

  • Aquatic plants are the biggest food competitor for algae
  • cover about a third of the water surface with plants
  • particularly suitable chickweed, waterweed, frog bite, crab claws, duckweed
  • Thin out plants from time to time due to strong growth
  • Clippings can be placed on compost
  • regular fishing of the algae using a special algae net
  • Algae easily compostable
  • ideally, the pond will be in equilibrium

fish and other aquatic animals

Fish and other small aquatic creatures also work like a biological filter.
However, it should always be ensured that the number of animals is limited. If possible, they should feed on the natural food sources in the pond (e.g. algae). An additional supply of fish food would in turn increase the nutrient content of the pond.

  • for larger ponds grass carp, silver carp or koi carp
  • Caution: they also eat smaller fish and soft pond and underwater plants
  • for smaller ponds rudd, goldfish
  • also use of water fleas, European freshwater shrimp and pond mussels
  • Snails are also algae eaters (swamp snails, ramshorn snails, pointed mud snails, small mud snails)
  • Water fleas have already eaten up all the water turbidity by mid-May
  • in addition, in fish ponds, a filter system is required to dispose of excess nutrients
  • Install additional technology, especially with large fish
  • do not use animals from local waters (nature conservation)

pond cleaning

The most effective way to combat algae in the garden pond is without a doubt a complete cleaning of the small body of water. However, a few things should be taken into account:

  • not just exchange water
  • clean all utensils thoroughly
  • Remove a layer of sludge from the pond bottom
  • Replace old pond soil with new, low-nutrient substrate
  • Use of nutrient-poor sand (as little as possible)
  • Prune plants vigorously and divide if necessary
  • place in new nutrient-poor pond soil and special plant baskets or embankment mats
  • specially tested tap water suitable for filling
  • sometimes tap water is enriched with up to 5 milligrams of phosphate per liter (protects lines from corrosion)
  • if they contain a lot of phosphate, use a phosphate binder
  • more suitable groundwater, is low in phosphate
  • rainwater would be ideal

tip: Smaller ponds can be covered with small nets in autumn. As a result, no leaves get into the water.

mineral binders

These remedies offered by specialist dealers do remedy the symptoms, but not the cause. Flocculants for quick clarification of the water are very popular here
phosphate binder:

  • is not absorbed by algae or other aquatic plants
  • bound to phosphate by chemical processes
  • thus deprived algae of food basis
  • different means available
  • either sprinkle the agent directly into the pond water
  • insert others directly into filters
  • harmless to fish

Algicides - algae killers

Here, too, different means are available in specialist shops. Although the use of these agents quickly clears the water, the causes are not eliminated. However, with long-term use and the wrong dosage, these agents are harmful to fish and other microorganisms.

  • clumping of the algae occurs partially
  • Then record through the filter
  • other remedies contain monolinron or copper sulfate
  • Use only with exact knowledge of the pond volume due to correct dosing
  • do not use permanently
  • if the water values do not match the recommended values - the remedy has no effect

tip: It is recommended to measure the visual depth in the pond. This provides information about the success of the algae control. To do this, dip a folding rule into the water until you can no longer see the tip.

prevent algae formation

A lot can already be done when creating a new garden pond so that algae do not have the opportunity to spread en masse in the pond. A few factors must be taken into account:

  • choose the right location
  • Pond in a depression unsuitable
  • Mineral fertilizers and garden soil from the surrounding area can get into the pond
  • better place on a slight rise
  • not directly under trees (leaf fall - nutrient enrichment)
  • a third should be in the shade
  • Pond surrounded by a 60 cm deep drainage ditch
  • fill this with coarse-grained construction sand
  • create different levels (swamp zone, shallow water, deep water)
  • use suitable plants in the individual zones
  • Note pond size
  • must be large enough that water does not heat up as quickly
  • the smaller and flatter, the more common algae problems
  • Minimum depth one meter, preferably even deeper
Water inlet of a garden pond

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