Green Pond: How To Fight Algae With Milk

To rid the pond of old, you do not have to resort to chemical means. An old home remedy to fight algae is cow’s milk. Especially in natural ponds, this is an effective alternative to chemicals.

To the point

  • Lactic acid bacteria can reduce algae
  • Cow’s milk contains only small amounts of lactic acid bacteria
  • Cow’s milk can promote algae growth due to other ingredients such as fats or lactose
  • Alternatives, such as whey, have a better mode of action
  • Check pH value constantly to prevent over-acidification of the water

Causes of algae


Algae form in the pond when the balance is no longer correct. Usually there are microorganisms responsible for the self-purification of the pond, which included lactic acid bacteria. However, if the nutrient content in the water increases, combined with heat, these organisms are eventually overwhelmed and the water tilts.
If the nutrient content in the water is too high, algae begin to multiply abruptly. Especially stagnant waters are often affected by this and filamentous algae and floating algae multiply in them. A small amount of different types of algae does not harm the inhabitants in the pond, but if the load of algae is too high, this can lead to turbidity of the water, from which not only the pond inhabitants, but also aquatic plants suffer.

Note: The turbidity in the water reduces the incidence of light. As a result, aquatic plants can perform less photosynthesis and die in the long term.

Use lactic acid bacteria

Lactic acid in concentrated form is already available in specialist shops as a means of combating algae. However, commercial cow’s milk contains only a few lactic acid bacteria and is therefore only effective to a limited extent. Occasionally, milk even exacerbates the problem because the protein and fats it contains can be additional food for algae species.
Raw milk has a somewhat higher content of lactic acid bacteria. The content of lactic acid bacteria can be additionally increased if you let the cow’s milk stand in the warm for a few days.

Using milk

The use of cow’s milk as a means of combating algae is controversial in terms of its effectiveness. Therefore, you should always proceed with caution when using cow’s milk.
Preferably use raw milk, which you apply as follows:

  • Allow raw milk to stand in the warm for one day.
  • Skim fat off the surface of the raw milk
  • Pour raw milk into water in a ratio of 1:2500
  • Stir raw milk lightly

For a small pool, usually already one liter is more than enough. If you only use a water barrel or an even smaller vessel as a mini-pond, you should add a maximum of one small glass of raw milk to the water. With such small water containers, it is sometimes even more sensible to approach the ideal content with the smallest amounts. This also applies if you do not know exactly how much water your pond holds.

Alternative dairy products

Instead of using cow’s milk to fight algae, it is more advisable to use dairy products with a higher concentration of lactic acid. An alternative is sour whey, which is produced when milk coagulates with the help of lactic acid bacteria. The advantage is that there is practically no fat left in the whey and the lactose is also much lower. On the other hand, the acidity is higher, which may have a negative effect on the pH value in the pond.
When using whey, proceed as follows:

  • Check pH value
  • add whey if the pH-value is higher than 8
  • mix whey in the ratio 1:2000
  • check pH-value again
  • should not fall below 7.5
  • check again after 2 days
Bei ALgenfall den pH-Wert überprüfen

The algae bloom is favored, among other things, by a very high pH. Through the acid whey you can not only fight the water algae, but at the same time lower the pH. By doing this, you can also reduce the growth of the algae. Often this is even enough to combat the algae bloom.

Note: One advantage of whey is that it is not cloudy like cow’s milk, but almost clear. This does not add to the reduction of light in the water.

Frequently asked questions

Can I use milk from other animals?

Yes, lactic acid is also present in sheep’s or goat’s milk. You can therefore also use other animal milks.

How warm does the water have to be?

Lactic acid bacteria need a temperature of at least 8°C to be able to multiply.

How often can I use dairy products against algae infestation?

Wait until the next application at least until any cloudiness in the water caused by the cow’s milk has disappeared. Also, observe if there is any improvement or worsening from the milk.

Can I use lactic acid to gradually optimize the pH value?

Yes, it is actually much better for the organisms in the pond if you gradually lower a pH that is too high.