If the pool water is green, algae have diligently multiplied. A refreshing bath is no longer a pleasure, nor is it advisable. So that the water becomes clear again: Find the causes, initiate measures!

Causes of algae growth

Every pool water always contains tiny little algae. But the quantities are usually so small that they remain “invisible” and harmless to us. The regular use of chlorine and, in the case of larger pools, a filter system should also ensure this. Nevertheless, it happens every now and then that algae multiply explosively and turn the water increasingly green. In such a case, ideal living conditions meet missing or insufficient measures to counteract the so-called algal bloom.

  • very warm pool water
  • high nutrient concentration (due to leaf fall, sunscreen, etc.)
  • a pH value that is too low or too high
  • chlorine concentration too low
  • filter runtime too short
  • dirty or defective filter

Check pool water condition

If the pool water is green, the owner can not only assume that there is a lot of algae. The concentration of bacteria and germs is often alarmingly high. In addition, there are slippery pool walls with the associated risk of slipping. Therefore, green water is not a purely visual problem. Also because the condition can or will continue to deteriorate, immediate action should be taken. If there is only a green shimmer, the water can remain in the pool. There is a good chance that the measures taken will make it clear and free of algae again in a short time. If the water is already dark green, it is better to replace it and clean the pool thoroughly. After that, however, the causes must be eliminated, otherwise it will not be long before the water turns green again.

Stop algae waiting

With an algaecide, an algae protection agent, you can kill green algae, but also other types of algae. The amount required for this depends on the size of the pool. It is best to follow the manufacturer’s instructions. Although the water does not become clear due to algaecide, further reproduction is slowed down or even stopped for the time being. This step is recommended if the actual elimination of the cause may take some time. For example, when a defective filter pump needs to be replaced.

Measure pH and adjust to ideal value

Excessive growth of algae can cause the pH of the pool water to rise. Up to a value of 10 is the speech. However, the pH value in the pool water should be between 7 and 7.4. Only then can the chlorine used develop its full effect. Therefore, you should first determine the pH before turning to the chlorine itself in the second step. Proceed as follows:

  • Get a pH test kit or test strips and measure the value
  • Compare the measured value with the target value, determine the difference
  • if necessary, use a pH raiser or pH reducer (pool supply store)
  • Put the granules in the water
  • ensure even distribution (e.g. run water pump)
  • Check the pH value after a few hours
  • if necessary, improve until the value is optimal
Tip: If you want to be quick, you can also achieve small changes in the pH value (in exceptional cases) using simple home remedies. The pH value is lowered with vinegar and increased with vitamin C or washing soda.

Measure chlorine content and carry out shock chlorination if necessary

The optimum chlorine level in the pool is 0.5 to 1.0 mg/l. With heavy pool use and hot weather, this value falls by itself. Heavy rainfall can also lower the concentration, after which the algae have it easier. Measure the pH with an appropriate test strip. You may also have an electronic meter. If the water is very green and polluted, optimizing the chlorine level is not enough. A so-called shock chlorination must be carried out here. A much larger amount of chlorine is used than is otherwise usual. Be sure to follow the package insert and distribute the chlorine evenly over the filter system.

Note: For your protection, be sure to wear protective clothing, gloves, safety goggles and a breathing mask when performing the shock chlorination because chlorine is poisonous.

Remove coarse dirt

Organic pollution, such as leaves, increases the nutrient content of the water as it decomposes. These nutrients are fertilizer for the algae. If the supply of nutrients is large, their increase is only a question of when and to what extent other favorable living conditions are also present. While the dirt floating on the surface can be collected with a fine-pored net, dirt particles that have sunk or stuck to the walls must be cleaned with a vacuum cleaner or a pool brush.

Check filter and use against algae

Larger pools must be continuously cleaned of the finest dirt particles or organic material using a suitable filter system. If this doesn’t work because the pump is defective, clogged or the output is too small, the water is not clean and algae growth is promoted as a result. The filter runtime is often set too low. Depending on what your check reveals, possibly with the help of the device instructions, appropriate measures must be taken so that the pool water is optimally filtered in the future: For example:

  • clean filter
  • Repair or replace filter system
  • increase runtime
  • if necessary, use a larger filter system

The pump has an important task not only during ongoing pool operation, but also when concretely fighting the green pool water. It has to filter even more, for example the dead algae. If these remain, they form the ideal nutrient basis for renewed algae growth. Therefore, let the filter system run longer until the water is no longer green. Don’t forget to clean the filter inserts again after a shock chlorination or to backwash a sand filter system.

Note: If an electronic filter system is defective, its repair should only be carried out by persons with the appropriate specialist knowledge.

First wait and test and then swim

The bathing fun has to wait until the measures introduced have eliminated the causes and brought the desired result. It takes time for the chlorine to take effect and for the filter system to filter the water clean. The progress is visible as the pool water is less and less green and cloudy . But that doesn’t mean that you have ideal bathing water in front of you. Before using the pool again, be sure to measure the chlorine level, because it must not be too high, otherwise it can irritate the skin and eyes. The pH value should also be checked again and corrected if necessary, as it may have changed in the meantime.

Prevent renewed algae blooms

As soon as algae find ideal growth conditions, they immediately begin to multiply and the water turns green. It shouldn’t come to that a second time.

  • Measure pH weekly
  • in summer even several times a week
  • also measure the chlorine level regularly
  • take optimization measures if necessary
  • perform shock chlorination when replacing water
  • Clean the filter pump regularly
  • occasionally use flocculants to bind phosphate
  • Fish out leaves promptly
  • Cover the pool when not in use for a long time
Note: A water temperature of around 27 °C promotes algae growth enormously. In the summer, a properly chosen cover can help shield the pool water from the sun’s rays.

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