Darkened Aquarium Photo: Denissa
The purpose of the darkening is to allow the algae to die off in the event of heavy algae infestation, especially blue-green algae, by depriving them of light for several days. This reduces the algae’s lead over the plants and creates more favorable starting conditions for the plants. However, lasting success can only be achieved if the causes of the algae infestation are eliminated at the same time by other measures. Then the blackout is a good measure to initiate the fight against algae. In the long run, plant growth must be strengthened in such a way that the plants take away the necessary nutrients from the algae.
To darken the aquarium, you can mask it with cardboard or simply place a blanket over the tank. During the blackout period, feeding is not carried out, or only very sparingly, and any existing CO2 supply is switched off. The plants only receive trace elements, but no complete fertilizer. In the case of robust plant species, the addition of nutrients can be completely dispensed with.
The necessary duration cannot be determined with certainty. In some cases a few days were enough, in other cases two or even three weeks had to be blacked out. The only thing that is certain is that the blackout must be repeated if it ends too early.
During the blackout, additional oxygen is brought into the aquarium with an oxygen stone. The remains of dead algae and plant leaves are decomposed by oxygen-consuming bacteria and fungi. If there is not enough oxygen, anaerobic bacteria take over the decomposition. These bacteria do not require oxygen and reduce the dead organisms to toxins (ammonia, sulfur compounds, etc.) that smell foul. These toxic compounds kill fish and plants. If the water stinks despite the oxygen supply, the sludge must be vacuumed off urgently.
Whether and to what extent plants are damaged by the blackout depends on the sensitivity of the plants and the duration of the blackout. For example, survive B. Hornwort, Vallisneria, Rotala rotundifolia and Java moss also a two-week blackout, while Barclaya longifolia and Echinodorus osiris are clearly damaged. Damage must also be expected with cryptocorynes due to their general sensitivity. Even if more sensitive species sprout again after being damaged by the blackout, care must be taken to ensure that the other plants do not get such a great growth advantage that the sensitive species die again in the long run.
Fish are not harmed even by a longer period of darkness. In the case of sensitive fish species, however, the pH value should be monitored.
Per arguments
- A good first step is to achieve a largely algae-free aquarium.
- Practical experience confirms the effectiveness of the blackout.
- Plants and fish are usually not harmed.
- The proliferating blue-green algae may be mutations of ordinary, slow-growing blue-green algae and are particularly sensitive to light deprivation.
Contra-arguments
- Plants weakened by algae are further damaged by deprivation of light.
- The necessary duration of the blackout cannot be determined in advance.
- Plants and fish could be harmed.
- Lasting success can only be achieved through good plant growth.
- Symptoms are treated, but the causes are not eliminated.