The so-called blackout treatment is a relatively easy non-invasive measure to rid an aquarium of algae. It is efficient against blue-green algae (cyanobacteria), most filamentous green algae and algae films. This method is almost totally inefficient against red algae, though.
During a black-out treatment, the aquarium is shaded off against all light during several days. You should not only switch off the aquarium light but also prevent sidelight from entering the tank. Dark cardboard or pieces of cloth are a good means of shutting out all light. During the black-out it is very important to keep the tank well-aerated. Simple measures that help move the water surface more intensively are for example setting the filter outlet higher up or installing an airstone in addition. Make sure you keep the organic load in the water down and feed only very sparingly. Shut off your CO₂ supply, and abstain from adding liquid fertilisers. Remove the algae as thoroughly as possible manually or by siphoning them off before you start the black-out treatment.

The lack of light weakens all organisms that rely on photosynthesis. As a rule, aquatic plants can deal with this treatment a lot better than algae. Only black-out your tank as long as the algae need to disappear. The use of algae-eating aquarium animals can speed up this process considerably. A maximum timeframe would be two weeks, given that your plants mainly belong to the shade-loving group (mosses, ferns, Anubias, Cryptocoryne). If you happen to have many stem plants and sun-loving ground-covering plants in your tank, keep the black-out period to 7 days max.

During the treatment, you ought to peep into your tank every three to four days to check on the algae and the state of your plants and animals. For feeding your aquarium inhabitants you can switch the lights on briefly.

After the completion of the black-out treatment you should bring your aquarium back to its original state - with the exception of the algae, of course. Do a large water change, as the dying algae can pollute the water considerably. The stem plants may look straggly and a little leggy after the treatment, which is due to the lack of light. After some time under regular light they will regain their old appearance.
Important: a black-out treatment will remove the algae rather easily, however, it does not remove the reason why the algae proliferated. It is really important to find out why this happened in the first place, and to take countermeasures, or else there may be another algae outbreak. Algae growth is often furthered by a nutrient imbalance, and for this reason it is important to check the water parameters of the aquarium and to optimize the fertiliser regimen for the aquatic plants. Quite frequently the reasons for algae lie in the macronutrients (NPK) or in the carbon dioxide (CO₂) supply. For a better basic understanding of algae in the aquarium we recommend you read this article.