All About Aquarium Lighting - Part V
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All About Aquarium Lighting - Part III

An Article by George J. Reclos

continued from previous page..


After a six-month period of use, a fluorescent tube will emit approximately 60% of the initial light intensity. Switching on and off is the factor which is mostly responsible for this decrease. The same effect is also observed for Metal Halide lamps. Reducing the number of switching on and off the lights increases the life span of the lamps. You can achieve that by using a timer that will switch on and off the light on a regular basis. Using an alternative light source when working in the tank after the lights are off, or before lights are on, you avoid the fast “ageing” of them. More problems with switching on and off are associated with Metal Halide lamps. Never try to switch on a lamp of this type, if the lamp is not completely cooled, after eight or ten hours of function.

It is recommended to change the tubes or bulbs every six months, to maintain the same amount of light year round, especially if you have keep plants and/or invertebrates.

Some of the sodium and mercury vapor lights are unsuitable for lighting reef aquaria. Also avoid the HQL and HQI – NDL lights as their spectra and color temperature (4300° K) are not suitable. Nevertheless, if one manages to block the ultraviolet emission they can help invertebrates to grow as this type of light source gives a lot of bright light. The quartz halogen lamps, although they cost a little, are also unsuitable for aquariums mainly because of the tremendous heat and low color temperature.

A good quality reflector may increase the quantity of light in the tank by up to 50%.

It is also recommended to clean the light bulbs/tubes from time to time. This is more requisite when the lights are placed near the water level, so water splashing fills them with salts and other depositions, which reduce the quantity of light emitted. Before cleaning turn off the lights and let them cool enough for handling. A piece of cloth moistened with distilled water is perfect for this job.

Metal Halide lights should not be placed closer than 30 cm to the water surface, or they will overheat the tank.


There are many terms used in this article or found on the lighting equipment you buy for your aquarium. Here is a short glossary, which will help you identify those terms and understand their meaning so you will be in a better position to make your selection. Please note that these terms are universally accepted and mean the same in all countries.

Light (visible)

Visible light is that part of the electro-magnetic spectrum that lies between the wavelengths of ultraviolet (380 nm) and infrared (700 nm). 

Light (invisible)

This consists of wavelengths which are not "seen" by the human eye. Of course, this doesn't mean that the rest of the animals are not able to see in those regions. The most known regions are the infrared and the ultra violet ones, which happen to be below the red and over the violet rays respectively. The UV rays are useful in sterilizing the water.


This is an indication of the power consumption required by the lighting equipment you use. Two light sources requiring the same power may produce different levels of light. Usually the energy, which is not used to produce light, is released in the form of heat. For the aquarium purposes, heat is undesirable so the hobbyist should opt for those solutions that produce more light / watt consumed. Fluorescent tubes and Metal halide lamps make good use of electricity power, while incandescent and halogen lamps do not.


This is the total amount of light a bulb is capable of generating and is perhaps the most important information you need to know from the manufacturer. If we have two light sources emitting in the same spectrum then the one, which emits more lumens, will be definitely brighter. However, for the aquarium hobbyist this is not absolute. A lamp may produce many lumens but be poorly focused (which means those lumens will never reach your plants or corals) or emit the wrong wavelengths or bands (green band for instance, instead of the red / blue you need).


This is the actual intensity of the light falling on a specified area and is defined as lumens per square meter. Which means that, if all the light from a 3000 lumen lamp was perfectly focused on a 1 square meter area, the light intensity at any spot would be 3000 lux. It is obvious that this is a much better way to express the lighting requirements. However, the amount of light that falls on your gravel or on the leaves of a specific plant in your tank is something you will have to measure. The manufacturer doesn’t know the depth of your tank, the presence of reflectors etc. so he can’t give you this figure. The difference between the Lumens and Lux is that Lumens are emitted while the Lux are Lumens that reach a specific surface.

CRI (Color Rendering Index)

The color rendering index identifies the degree of color shift objects undergo when illuminated by a particular light source. In simpler terms, the CRI expresses the degree to which a light source renders the true color impression. The CRI is an index and ranges from 0 to 100. A light source having a CRI of 100 means objects illuminated by it look like they're supposed to; that is their natural color is not distorted. A light source having a very low CRI would tend to make objects appear to be a different shade or even colour that they really are. An example of light with a high CRI is, obviously, sunlight. Some fluorescent tubes have a very high CRI (upper 80s or low 90s).

Kelvin temperature (Light color)

White light can have different "warmth". A bit more red/yellow and white light appears "warmer". A bit more blue and light appears "cool". This can be quantitatively assessed by assigning a colour temperature, given in degrees Kelvin. Think of colour temperature as the colour of a block or iron (a black body) as it is heated to various high temperatures. A warm, reddish light is around 3500 degrees Kelvin, and above 6000 degrees Kelvin the light takes on a bluish tone. Sunlight is somewhere around 5000 degrees Kelvin. Which means that from the physicist’s point of view, blue is “hotter” than “red”. The “zero” in the Kelvin scale is the “absolute” zero which is a theoretical value (can’t be reached). A body, which is brought to this temperature, is assumed to emit no radiation at all.


This describes the wavelengths of light that make up the light source. Visible light (see Glossary) is a continuous band of colors ranging from violet to red (380 – 700 nm). Sunlight and incandescent light is composed of all visible wavelengths. Fluorescent and metal halide bulbs emit only a few wavelengths (or bands) depending on the phosphors or rare earths they contain.

Light as an electromagnetic wave 

Light is something strange. In physics it can be interpreted as both a particle (named a “photon”) or a wave. Thus, it has the properties of both a particle (can fall on something, change course, will bounce off an obstacle etc.) and a wave (has a wavelength, period, frequency etc.). The wave “form” of the light is the most interesting one. Thus, the wavelength it the length between two peaks of the light wave (like the waves in the sea) while the frequency is how many such waves are sent per second. Light will travel 300.000 Km / second, no matter what its wavelength is. This means, that if it is short, more waves will be sent in a second, while if it is a long one, less waves will pass in a second. Thus, the longer the wavelength (red) the lower the frequency. The shorter the wavelength (violet) the higher the frequency. The energy, which is “carried” by a photon, is proportional to its frequency. As a result, the violet rays carry more than double the energy the red ones carry. This is of outmost importance for the photosynthesis of the plants and corals since they need the high-energy photons.

Loss of intensity

As we move away from the light source, the intensity of light drops geometrically. Thus, at double the distance only one fourth of the light intensity is available. In deep tanks, the light intensity at a depth of 80 cm is 1/16 of that at 20 cm. This calculation is true for air only. With water, even more light is lost during travel but the above calculations will give you an estimate on how many more lamps you must use.


Zooxanthellae are a type of din flagellated (they form a couple of visible flagellates on them) micro algae, that use the tissues of some species of invertebrates, like corals, sponges and clams, to live in. In return for this symbiosis they provide food and oxygen to their hosts, while they consume the excess of Carbon dioxide, Nitrogen and Phosphorus that invertebrates produce.


Photosynthesis is the operation of the plants in which light is used as the energy source to produce food (sugars). During this process, plants consume Carbon dioxide and release Oxygen while they “store” the light energy in the sugar molecules. This phenomenon can be visible in planted aquaria, ponds and lagoons, after some hours of lighting, in the shape of tiny bubbles coming out from little pores on the plants’ leaves. The reaction is reversed in the dark. In the  dark the plant will produce carbon dioxide and consume oxygen thus utilizing the energy which was stored in the sugars.

Acknowledgements: Many thanks to Andreas Iliopoulos for critically reviewing this article. His precious comments added a lot to its integrity.

Reference: The Kelvin rating chart was obtained from THE REEF AQUARIUM (vol.1) by Charles Delbeek and Julian Sprung (Ricordea Publishing 1996).

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