Algae in the pond

Why are algae a problem?

Why are algae a problem? Which algae species can be found in the pond and how can you combat them effectively in the long run?

Algae belong in the pond as much as other water organisms. You can’t easily prevent their occurrence. As long as the algae growth remains low or it doesn’t noticeably gain the upper hand there is no need to worry.

Strong algae growth, however, has a dramatic effect on the water: Algae are plants and produce oxygen during the day (which is a positive effect). But at night the situation reverses and the algae consume oxygen. With a heavy algae infestation the oxygen can drop into dangerous ranges at night and threaten the life in the garden pond. It is then advisable to aerate at night.

Exposed to light (during the day), the algae continue to consume carbon dioxide (CO2) like the water plants. This consumption can lead to an extreme increase in the pH level (acidity > 9), which in turn leads to a life-threatening environment. At night they, like all plants, also produce carbon dioxide, leading to a lower pH value.

If the CO2 content in the water is no longer sufficient, algae are able to dissolve the CO2 out of the carbonate hardness (KH). The result is a decrease of this important water parameter which leads to an unstable pH level. The pH level drops significantly at night (mostly to values as low as 6) and rises during the day to far too high values (up to values over 10). This can create huge problems for all the life in the pond.


A change of one step in the pH value means a tenfold of the lye amount (pH value increases) or the acid amount (pH value decreases). A change of two steps is a hundredfold change of concentration, 3 steps imply a thousandfold change of concentration.

Finally it is worth mentioning that dying algae are broken down by bacteria during oxygen consumption. Therefore the water needs to be aerated additionally when algae die, either by themselves or when combated actively.

These processes seem to be very complicated at first glance. But don’t worry: If you approach them step by step and break them down into single processes, they are easier to follow and absolutely manageable – we promise!

Which algae species can be found in the pond?

Two groups of algae commonly occur in garden ponds:

Thread algae and floating algae belong to the green algae species and are always a result of an excess of nutrients in combination with too much light. They therefore usually appear in spring and summer, when the intensity of the sun is increasing or is very strong.

There are temporal correlations with the growth of the higher aquarium plants. In early spring, when the light quantity intensifies, the floating algae primarily develop because the nutrient competition provided by water plants isn't there yet. When the floating algae have finished blooming, the second step commences. This is when the thread algae appear with the higher aquarium plants in early summer/summer because more light is falling on the substrate and on the water plants. While this phenomenon does not entail a 100% coupling of the occurrences of the respective algae group, many interdependencies do exist, including an interdependency between algae occurrence and the respective meteorological conditions (cold/warm spring).

algas filamentosas

Filamentous, greenish or brownish algae which stick to the substrate, the borders of the pond, the stones and plants.

Floating algae

Floating algae which turn the water green. To most people this term is unfamiliar and they just call this problem “green water”.

Blue-green algae

Another „algae species“ is the so-called „blue-green algae“. They form a smeary blue- green or brownish algae coating (together with diatoms) on pond walls, substrates, stones and even plants. The blue-green algae are not really algae but bacteria (cyanobacteria), which use the light as an energy source. They are something between animals and plants. The blue-green algae occur with high water pollution and when organic matter (leaves) decays and gets broken down.

Other algae species in the garden pond, such as red algae, play a minor role. In general the garden pond enthusiast hardly notices them. It is also important to know that algae can absorb huge quantities of nutrients and store them in their cells. Thus many algae species can absorb and store more than the 100,000-fold of the concentrations in the water, some algae species even up to the 3.8 million-fold of the measurable concentration. The algae use this nutrient buffer for their unimpeded further growth when there are nutritional deficiencies. This is why we can frequently observe algae continuing to grow vigorously, even though we cannot detect any nutrients in the garden pond.

How do you combat algae in the pond?

There are, of course, JBL anti-algae agents designed to successfully combat the various species of algae. But this success could be short-lived: because when the algae combatted by the anti-algae agents die, they release the nutrients they have stored in their cells, and these nutrients serve as food for the next generation of algae. For this reason you need to embark on this long-term and reliable algae control in 3 steps:

  1. Stabilise the water to ensure that the anti-algae agents have an effect at all.
  2. Target the specific type of algae.
  3. Bind the nutrients so that the algae can no longer grow.

To give you precise instructions on how to combat your algae effectively we’ll need some details about your pond. Have a look at our pond laboratory!

Laboratório/calculadora para lagos

This worldwide unique pond laboratory gives you dedicated support to combat problems with algae and cloudiness in your pond.

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