Light in aquariums has a dual function.
1. We require suitable lighting to enable us to properly observe fish and plants.
2. Function of lighting is fundamentally related to the aquarium's biological and chemical processes.
3. Light in an aquarium has an essentially vital and life-sustaining role to play, but can also if applied incorrectly cause severe negative consequences.
That is why lighting needs to be carefully and properly controlled.
Light for observation purposes:
The most obvious function of aquarium lighting is to properly and most effectively reflect the colors of both fish and plants. Even the term "properly" is itself subjective in that the Aquarius will possibly experience light differently to fish. And, as we will discover later, plants themselves prefer very specific spectral colors for their own growth.
The biological aspect of aquarium lighting:
This aspect of lighting is even more difficult to illustrate. It is closely interrelated with the biology of aquarium plants. Light produces energy for the plants, required for their metabolism, ranging from nutrient assimilation to chlorophyll production. Light is the instrument by which biological processes can be enhanced or slowed down, a process comparable to acceleration when driving a car: if one presses the accelerator pedal, the vehicle will speed up. The stronger the light in the aquarium the quicker the plants will grow, resulting a increased consumption in CO2 and other plant nutrients. Nutrient deficiency can be a likely result if an adequate and exact re-supply is not catered for. On the other hand oxygen production, too, is increased through light. Not only do fish require an optimum oxygen supply but the bacteria do as well, to enable them to mineralize the aquarium's waste {excrement, urine and waste products from food, algae and plants}.Not only the quantity of light but its quality, too, plays an important role in the aquarium's lighting setup. By that we mean the spectral composition of light. It is common knowledge that white light consists of a broad spectrum of different colors, from blue via green and yellow to red. There are two areas within the color spectrum in which the plants' assimilation reaches maximum levels. On the one hand it occurs in the blue, on the other hand in the red range of the spectrum. This differs from human vision, where the highest degree of sensitivity lies within the yellow-green range, a field irrelevant for plants. A botanist named Peffer was already able to prove at the turn of the century that the different light waves influence the plants' assimilation process. The assimilation of green algae, for example, in spectrally dispersed sunlight was best within the red range. This is perceptible by bacteria concentrating where oxygen emission is strongest. In brief: light is of essential importance for the beautiful and healthy development of aquarium plants, but light can also kill them if applied incorrectly. It is also important to consider that the aquarium is basically a very small but also diverse biosphere in which extreme conditions are rapidly reached and also exceeded. It is therefore hardly useful to just throw any recommendations regarding light at the Aquarius, calculated on the basis of Watts per litre or cm of aquarium volume, without considering other growth factors in the aquarium. Under certain conditions 500 Lux can be too powerful, in another case 5000 Lux might be insufficient.
Considerations regarding location:
In an aquarium we usually cater for plants from the most diverse regions of tropical waters. The plants are, however, fundamentally different in species and genus. Normally we would introduce light seeking plants, which in there natural environment would grow on banks or near the waters surface. For example, small Echinodorus- and Anubia species, but also Lilaeopsis and Marsilea - to the deepest and therefore darkest area of the aquarium, serving as what is called foreground plants. Shade-preferring plants, e. g. many Cryptocoryne species, are often found under bright aquarium light. In other words: sunlight- or shade-preferring plants from Africa, Asia and South America are arranged without due consideration of their inherent needs with regard to light. Fortunately, though, water plants have an inherent capability to adapt to overall light conditions. This ability differs amongst the plants. In scientific terminology it is called chromatic adaptation. It enables many aquaplants to adapt to the specific light conditions existing at its location. We can assume that those plants successfully kept in today's aquariums have developed this capability the best.
Balance of light and nutrients:
The most obvious problem facing an aquarium is balancing the supply of energy and nutrients. I can remember how many aquarist's encountered huge problems with the introduction of fluorescent lamps. Until then aquariums were kept pretty dark, with the new lamps, however came not only more light but a lot more energy as well into the tank. Initially it was not realized that it is problematic to introduce more energy into the aquarium and simultaneously not to increase C02 and nutrient supplies. Increased light energy accelerates the plants' assimilation and metabolism, driving up C02 and nutrient consumption. The resulting nutrient deficiency is far more detrimental to the plants than the previously existing lack of light. The plants will die.
Light influences the chemical composition of water:
Light quantity also impacts to a high degree on the chemical composition of water. This is not least due to the relatively small volume of water. For example carbon dioxide {C02}: the following process in the aquarium is referred to as "biogenetic decalcification".
When the free carbon dioxide gas, dissolved in water, is consumed through light and plant assimilation and is not replaced through C02-fertilization, plants will draw their further carbon requirements from carbonates {carbonate hardness}. The remains are clearly visible everywhere in the form of cal- careous rings, for instance near the water surface. The intensity of this process is light dependent. The more powerful the light the more energy the plant possesses to carry out this chemical process. Another consequence: the water's pH-level rises, depending on light quantity and plant species, possibly even reaching dangerous levels for fish. Elodea and Cabomba, for example, are capable to increase the water's pH-level up to 9 or even 10. Furthermore, under the influence of light plants are able to withdraw considerable quantities of salt from the water.
The light day in an aquarium:
Finally, I wish to comment on the often brought up question of how long an aquarium should be subjected to lighting. 12, 14 or only a mere 8 or 10 hours? Basically there is a very simple answer to this problem. In an aquarium we usually keep fish and plants originating from the tropics. There, over the course of the whole year, day and night are a consistent 12 hours each. The difference between "summer and winter time" is merely a few minutes. It is a little bit different, though, under water. Here the ecologically active dependency of the radiation from the sunlight's angles of incidence and refraction play a role. The lower the sun's setting, the higher the loss on reflection. According to W. Schmidt it already measures 13, 5 % on an angle of incidence of 700, 35 % at 800. And at 90%, when the light rays fall in parallel to the water surface, all light is reflected. In short, a day under water within tropical regions is much shorter than above the surface. We are well advised to adopt this as a guide line for the duration of lighting in the aquarium, since it will also save us a lot of energy costs. My recommendation therefore is to set out the aquarium's lighting period to 8 to 10 hours. By the way, scientific research has found that extending the lighting period does not increase the plant's assimilation performance. The performance curve will already level off prior to switching off the light.