Rather than using algaecide potentially causing harm to plants, animals, producing workers and the rest of the world

  * Lower PO_4 value to the minimum by adding pug to the filter. Usually takes effect (with 1 mg/l) after 48 hours. It's the initial nurition for the algae.
  * Provide less food which increases the PO_4 value (frozen food and others).
  * Turn on an oxigen pump if you have one since O_2 production of plants is lower because of less light getting through.
  * Move the aquarium away from direct sunlight impact (see https://pets.stackexchange.com/questions/12150/how-to-move-an-aquarium for things to consider)
  * Keep a group of [Daphnia](https://en.wikipedia.org/wiki/Daphnia) (more than 5 make sense) in a container with a separation/membran which allows water exchange and prevents fish from eating the Daphnia as well as them to get out into the aquarium (where they'd be eaten). Ideally the separation makes the Daphnia invisible for the aquarium inhabitants in order to not provide them moving food all the time. Place the container with the separation into a stream (of the filter outlet or close to the air pump). The stream is visible by the green algae swathes.

You can track the progress by checking how deep you can see into the aquarium day by day. The Daphnia should start to multiply by 4 every 2 to 5 days and 60 l are cleared after a week. Then decorate your Daphnia with orders and release them as living food.

Example glass with plastic kitchen sieve (since Daphnia can't fully control their movement chances that small Daphnia get through the sieve are small; Mollys don't show any interest in Daphnia; a test with a nylon stocking with a knot showed that too few water is exchanged in the container):

[![Daphnia container (glass with kitchen sieve)][1]][1]

This _might_ work with red and even blue bacteria algae as well since they're consumed by Daphnia.

After installing an undergravel filter in the aquarium (which was a severe action which can't be performed in all aquariums) I have cristal clear water and it might be that the problem is prevented for ever.


  [1]: https://i.sstatic.net/k1lcw.jpg