Blackout Treatment Guide to Fight Algae

The blackout treatment is a relatively easy non-invasive measure to fight algae issues in the aquarium. It is efficient against blue-green algae (cyanobacteria), most filamentous green algae and algae films however this method is almost totally inefficient against red algae.

The treatment involves the aquarium being shaded off against all light source for several days. Aquarium lights should be switched off and also measures to prevent sidelight from entering the tank should be in place. Dark cardboard or pieces of cloth are a good means of shutting out all light. During the blackout it is very important to keep the tank well aerated. This can be done by moving the water surface more intensively by either setting the filter outlet higher up or installing an airstone. It is important to ensure that the organic load in the water is kept down and feed only very sparingly. CO2 supply should be shut off and all liquid fertilisation stopped. The algae have to be removed manually as thoroughly as possibly by either trimming or siphoning them off before beginning the blackout treatment.

The lack of light weakens all organisms that rely on photosynthesis even aquatic plants however aquatic plants can deal with this treatment a lot better than algae. It is important to only blackout your tank for as long as the algae is needed to disappear. The use of algae-eating aquarium animals can help 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 period, no peeping into your tank i.e do no attempt to open and check on your aquarium. After the completion of the blackout 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 yellowish, poor and unhealthy 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 blackout treatment will remove the algae rather easily, however, it does not remove the reason why the algae appeared. 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.

Basic Steps to Performing a Blackout

  • Feed the fish one hour before starting the blackout They will not eat for the entire course of the process. Do not overfeed them, give them the regular amount of food.
  • Try to phisycally remove as much algae as possible. Use algae scrapers, a toothbush, trim heavily on badly affected plants and etc.
  • Do a 50% water change.
  • Switch off the CO2 for the entire course of the blackout.
  • Stop fertilizing your plants during the blackout
  • Introduce an air-pump or raise the inflow lily pipes to provide oxygen to the aquarium during the blackout.
  • Get a thick cardboard or plastic bag that will cover the aquarium without letting any light through - not even at the corners. Cover the top of the aquarium with something so the blanket will not fall into the water. (Evaporation can also make it wet...) It would be best if you could get a black wrapping foil or black garbage bin bags
  • Cover your aquarium entirely, wrapping it multiple times. Light should absolutely not get into the aquarium. Check all sides and corners.
  • Leave the aquarium like this for 5 to 7 days. Do not attempt to peek into the aquarium for the duration of the blackout!
  • After 5 to 7 days, remove the cover slowly, gradually and carefully, not to disturb the fish. Wait at least 30 minutes before switching the aquarium light on.
  • Clean the filter as the dead algae can clog it. Make sure you do not kill the bacteria in your filter by leaving it off for a long time, or rinsing the filter material in tap water that might contain chlorine.
  • Make a large water change and remove any remaining algae physically
  • Set your lights for 4 hours a day for the next week or so and increase lighting period over time once the algae doesn't reappear.
  • Start dosing liquid fertiliser normally and switch on the CO2 (initially adjusted to the 4-hour-lighting-period.)
Note that plants can also suffer because of the blackout and they might look different for some time. You will notice they have an accentuated growth upwards, trying to reach the light.