How long should an aquarium be lit? This is a very fundamental question in which the interests of the aquarium owner meet the needs of the animals and plants in the aquarium. Here we show you the most common options.

Naturally, an aquarium owner wants to be able to view and enjoy his or her aquarium as fully as possible at any time. You achieve this with the longest possible lighting period. However, the animals and aquatic plants in this ecosystem also have needs and requirements regarding the light situation. You should meet these for the well-being of the inhabitants in order to enjoy your aquarium for as long as possible.

Taking nature as a guide

In this respect, it does no harm to take our cue from the conditions in nature when it comes to the lighting period. A typical day in the tropics usually comprises a light phase of about 12 hours, during which the intensity increases from the morning, peaks at midday and decreases again toward evening. This near-natural daylight rhythm can also be transferred as a first guideline to lighting in the aquarium hobby. These days, energy-saving LED lamps are preferably used. For the dimming and twilight phases, we recommend using an LED controller with which you can program the intensity of the lighting as a daylight curve. This creates near-natural light phases that also blend into one another very softly and gently. Switching the aquarium lamp on and off abruptly can sometimes cause stress reactions in the animals. Especially in open tanks, there is a particularly high risk of fish even jumping out of the aquarium in panic.

Atmospheric lighting effects, such as playing with light colors, moonlight and sunrise glow, can also be implemented with such a controller and suitable lighting. These effects are not strictly necessary and in part go beyond the role of a purely functional light source, but they are certainly justified for creating different lighting moods for the viewer of an aquarium (and its inhabitants).

Planted aquarium

In an aquarium or aquascape with a focus on aquatic plants, lighting naturally plays a special role. Plants need light for their metabolism as part of photosynthesis, and it is therefore to be regarded, so to speak, as a "nutrient". Certain relationships between light intensity and the availability of CO₂ in the aquarium water must be taken into account. Plant growth is only possible when a so-called compensation point is reached, at which a certain minimum amount of light and carbon dioxide are available. These compensation points are not the same for all aquatic plants but depend strongly on the species. This is also why the trade in the aquarium sector divides aquarium plants into different categories, such as "low-light plants / shade plants", which need significantly less light. These include, for example, mosses and ferns. Typical stem plants as "high-light plants / sun plants", on the other hand, often have a higher light requirement. In between there is another category for the aquarium plants that get by with medium light.

Dependence of the photosynthesis rate in shade plants and sun plants on light intensity. Image author: Yikrazuul - Own work, license: CC BY-SA 3.0.

If you look at the framework conditions of Takashi Amano's Nature Aquariums, a maximum lighting time of 10 hours emerges for such a planted aquarium. In an aquarium layout with many stem plants, moderate to strong light intensity and a corresponding photosynthetic activity, you can observe after a certain number of hours that the plants at some point "close their heads". The foliage closes up, as if the aquatic plants were preparing for the dark night phase. The maximum of photosynthetic activity has been reached, and there is no further demand. In such aquariums, this point is often reached after 8 to 10 hours. It is therefore advisable here to adapt the duration of the aquarium lighting to this reaction of the plants. A period of 12 hours usually makes no sense in these cases, as it would only challenge the plants' metabolism unnecessarily.

In planted aquariums with good framework conditions and, above all, appropriate lighting, the photosynthetic activity of the aquatic plants can also be observed well during the day. After a certain time, the oxygen content in the water is so saturated that visible oxygen bubbles produced by the plants appear on the leaves and stems. This effect often occurs just a few hours after the start of the lighting period.

Lighting period & light intensity

The lighting period only indicates the pure time window in which an aquarium lamp is switched on. How strongly this lamp emits light is initially not related to it. That is determined by the light intensity of a lighting system. In aquarium practice, for LED lamps this is equated with the emitted luminous flux in the unit lumens. We have explained in this article which values define weak, moderate and strong light.

Sometimes you can try to make up for insufficient light intensity with longer lighting phases to get better plant growth. In itself this is not a bad plan, but as we mentioned above, plants do not grow if the compensation point of the respective plant species is not reached. Unfortunately, with an extended light phase and weak lighting, this point sometimes still cannot be reached. In these cases, the only thing that helps is either upgrading the lighting (and increasing the associated consumption of CO₂, micronutrients and macronutrients) or adapting the plant species to the respective light and nutrient conditions.

Lighting time during the cycling phase of an aquarium

When an aquarium has only just been set up and newly planted, the ecological system is still quite sensitive in the so-called cycling phase. The aquatic plants still have to get used to their new environment, and some have to switch from emersed to submersed growth. This process also affects their metabolism. It is therefore advisable to limit the lighting period of a planted aquarium with moderate or high light intensity to only 6 hours in the beginning. After about 2 to 3 weeks, the lighting time can be increased somewhat, for example to 7 hours a day. After that, the aquarium owner can extend the light phase by one hour each week until the desired target time is reached. The prerequisite is that the aquarium stabilizes, plant growth becomes established and an unwanted, massive spread of algae does not occur. If this is not the case and, for example, there is a heavy algae infestation, the lighting time should stay at one level and not be increased further.