All About Aquarium Lighting

All About Aquarium Lighting

Written by George J. Reclos Wednesday, 09 January 2002 01:00

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Article Index
All About Aquarium Lighting
Page 2: Light Sources
Page 3: Characteristics of Light
Page 4: Glossary
All Pages

 

Lighting is a key issue in fish keeping - I am sure most of you know that. Your particular set up may require too much or too low lighting, special colour lighting (colour temperature), special type of lighting (e.g. need for deep penetration), even lighting at special angles. All those elements can make a tremendous difference in what you and your visitors see when they look at your display aquarium and may also have a great impact on the well being of the animals and plants you keep in there. Of course, all those elements, when not explained in detail, create a much bigger confusion which forces hobbyists to opt for an "average" solution. What's worse, there are no rules of thumb and those that exist should not be used - another reason for more confusion. There are some principles though. Let's start by saying that the lighting requirements are dependent on the animals, plants and invertebrates which live in your tank and not on the size / shape of the tank.

Indeed, there are several issues related to lighting: Type of light source, amount of light (in Lumens or Lux but not Watts), light colour (more correctly: light temperature measured in degrees Kelvin), quality of light (CRI), duration of light and many more. Yes, things are not simple at all, when it comes to lighting. No wonder there is a whole chapter about it in our Physics books. I will try to make this issue as simple as possible but not oversimplify it. The aim of this article is to provide enough information for the hobbyist and enable him to choose the right lighting equipment because he knows what he is looking for. The article will have succeeded its goal if more and more hobbyists visit their pet shops asking for lamps with specific characteristics instead of relying on what is stock (or what the shop works with a greater profit margin). Of course, this is not going to be an "easy reading" article. It couldn't be - and it shouldn't be!

Light colour (temperature of light source). Our eyes (and of course the eyes of our fish and our plants' photosynthesizing elements) are "calibrated" to perform in ambient sunlight. Subsequently, our eyes interpret this light as white light. As all of us know, "white" light simply doesn't exist. In simple words, "white" is not a colour but a combination of many wavelengths (colours), which form a spectrum (see Glossary at the end of this article). What we perceive as "white light" is actually the average of the solar spectrum. This spectrum comprises roughly from six basic colours (and all their hues since the spectrum is continuous): Red, Orange, Yellow, Green, Blue and Violet. Those colours are listed in ascending frequency order (or descending wavelength, thus the Violet has the shortest wavelength and the highest frequency - hence higher energy). The visible wavelengths range from 760 nm (deep red) to 380 nm (violet) - see Diagram 1.

In the left diagram (Diagram 1, above) you can see the full spectrum of the visible light. Visual light is a tiny part of the electromagnetic wave spectrum. The right diagram shows the spectrum of sunlight. As you can see, the spectrum is continuous, while not all colours are emitted with the same intensity. However, all animals living on the surface of the earth or very shallow waters (sea or lakes) have light sensors (eyes or other organs) "calibrated" for this spectrum.

Any wavelength longer than 760 nm is in the infra red (IR) region while anything shorter than 380 nm is in the Ultra Violet region (UV). One very common misunderstanding occurs when we discuss the "temperature" of the light and the "warmth" of it. To the human eye, the "warm""cool" ones are at the bluish end of it. However, when we refer to light sources, things are exactly the opposite. Sunlight white usually has a temperature of 5000-5500 K while light sources rated at higher temperatures contain more and more blue in them (10000 - 20000 K) - see Diagrams 2 and 3 and Table 1.

Diagram 2 shows a full spectrum, low temperature light source. Note that the red / yellow region is enhanced while the emission in the blue region is reduced. This resembles the emission spectrum of an incandescent lamp.

Diagram 3 shows a full spectrum, high temperature light source. The red, green and blue regions resemble that of the sun. Compare this spectrum to the "bands" of the spectrum emitted by the full spectrum aquarium tubes (photos 2 and 3).

Of course, "white" is something relevant. We see the light coming from an incandescent lamp and the light coming from a normal fluorescent tube (the "cool white" ones) and we interpret both of them as "white", especially when we have nothing to compare them against. However, if you take a picture of those two light sources on a regular (daylight) film, you will see that the incandescent lamp is yellow while the fluorescent tube is green - see Diagram 2 and 4.

Diagram 4 shows the cool white fluorescent tubes found in most homes and buildings seem to be white while actually they emit a big amount of green light, which makes them unsuitable for the planted tank. This can be captured on daylight film.

This difference may not be crucial when reading a book, but it is very crucial for the well being of your corals or freshwater plants which need the energy of the red and especially the blue rays. Moreover, it is perfectly possible to create "white" light by mixing the three basic colours (red, green and blue). This creates a white light, although in this case the spectrum is not a continuous one but one which shows three peaks (or bands). This is the case in most good quality lamps. It goes without saying that, depending on the mixture of the colours a different result is obtained. Thus, the cheap household fluorescent tubes create a "green" cast with a narrow spectrum, the "full spectrum" lamps create a bright white colour, while the so called "actinic" lamps have a very narrow band in the blue section of the spectrum.

Some other factors that you should know (and perhaps use them to work for you) is that the shorter wavelengths (greens and blues) are less scattered in the water (and the air) therefore more penetrating while the red, orange and yellow rays are far more scattered (which is the typical answer to the question "why is the sky during the sunset orange?"). So, in an ideal, full spectrum lamp, more green and blue will reach the bottom than reds and yellows. Your plants will definitely appreciate that and the colours of the fish will be enhanced. However, you will "feel" that there is less light in your tank than actually is. In contrast, the red - yellow - green colour creates the impression of more light in your tank.

Moreover, the human eye "receives" different colours in different ways. If you had to take a look at monochromatic light sources (only one colour emitted) of the same intensity, then you would think that yellow is brighter than the rest and blue the dimmest. Keep in mind that the human eye is more sensitive to the green colour (hence the more green emitted by the normal tubes). Apart from that, red creates the "alert" feeling, orange the "warmth" feeling and blue the "cold" one. Thus we have two factors that make visual estimation almost impossible: One thing is what we see and another thing is how much light (and what type of light) reaches our plants, corals or fish.

Light is very important to the well-being of many organisms we keep in our tanks. Thus some of the organisms we use to keep in our tanks depend on light for their feeding needs. Corals, for instance, use the products of photosynthesis (food and oxygen), provided from the symbiotic zooxanthellae that live within their tissues. Zooxanthellae, on the other hand, use the by-products of corals with come in the form of carbon and wastes rich in Nitrogen and Phosphorus as their food source.

Light is also essential for the function of colour pigments of some organisms, while some other organisms use light to manage vitamins and/or minerals, useful for the constructing or maintaining structures of their skeletons.

Another critical fact for the right lighting of an organism is the angle that the specimen is placed in the tank. Some organisms prefer direct light, while some require that the light will "hit" them not directly from above but at an angle. Some others prefer to get the light obscured by shades. As already pointed, the species you will keep will determine the kind of light you need.

 

Light sources

There are many light sources available to the hobbyist today. One can easily name the sun, the incandescent lamp, the fluorescent tube (normal, HO, VHO), the metal halide lamps and even more exotic light sources.

Sunlight

The sun is of course the perfect light source. At least this should be the case - in theory. However, there are some inherited disadvantages, which make its use less than desired. The type of sunlight Northern Europe gets in the winter has nothing to do with the light falling on the Equator where our tropical fish and plants (not to mention the light sensitive corals) come from. The sunlight has an unpredictable duration (partially or totally absent in cloudy days) and of course leads to lots of algae since its intensity is much higher than what a normal tank can take. Yes, the Lake or river get the same kind of lighting but there are hundreds of thousands of gallons, hundreds of kilometres of water and great depths; all of them have nothing to do with a normal aquarium, On top of that, the sunlight increases the temperature. A tank lighted by the sun would normally experience great fluctuation of the temperature of the water, while the maximum temperature during the day may well be too high for your fish and plants. Bear in mind that, although the sunlight is white, this is only the visible part of its emission. The sun emits great amounts of infra red, ultra violet, x rays and even cosmic rays. However, the sun can be used in specially designed tanks as a complementary light source and is used as the only light source in outdoor ponds (in which case the algae growth, if kept under control, is used as a source of food).

Incandescent lamps

The incandescent lamp provides a cheap source of low quality light. However, in some cases, this is all you need. In this photo, a 100 Watt "spot" light is used with a 35 litre (10 gallon) fry raising tank with herbivore fish. The algae grows everywhere and is then eaten by the fish. During summer the duration of the lighting period is reduced to 4-5 hours daily since the heat produced would otherwise "cook" the fish.

The incandescent lamp is to be used only in emergency cases (sometimes I use them in my hospital tanks or even the fry raising tanks until I cycle a bigger tank - see Photo 1). This should be regarded as a cheap source of low quality light. The spectrum of the light produced by this type of lamps is continuous but it is biased to the red-yellow end of it. Thus, the light almost lacks the blue region, which is essential for most plants and corals. The advantages of this lamp are the low cost and the ease of installation. The disadvantages are the "yellow" cast, the tremendous amounts of heat produced and the low light intensity. This kind of lamp produces the least light per watt of electricity consumed, while most of the energy is released in the form of heat. While plants can't grow properly with this kind of light, algae can (and believe me, it will). The overall result looks too yellow but could be acceptable if there is nothing to compare against. Not really recommended. The normal bulbs produce a light with a temperature around 2700 K (red biased white) while the halogen lamps produce a light of 3000 K (a bit whiter but still red biased). The colour Rendering Index (CRI; see Glossary at the end of this article) of both types is 100. Please note that the more powerful the lamp the brighter it is. Thus, a 100-Watt lamp will be brighter (and thus more effective) than two 50-watt lamps.

Fluorescent tubes

Fluorescent tubes give out four times more light per watt consumed compared to incandescent light bulbs. There are many different types of fluorescent tubes. They differ in the physical size, composition of the phosphor and the wattage. When fluorescent tube is mentioned the standard T12 four foot (120 cm) tubes usually comes to mind. This tube has a diameter of 1.5 inches and is available in 18", 24" 36", 48", 72" and 96" lengths (lengths in inches since this is the way most lamps are sold in Europe, too). The T8 or "slim line" fluorescent has a 1" diameter tube and is available in 24", 36" and 48" lengths. T12 tubes are also available in U-shaped, that is a four foot tube is bent back on itself so it forms a large U, and is about 24" long. Circular tubes are available with several different radii, and in several different types. In the last few years, compact fluorescent tubes have become very popular mostly as replacements for incandescent bulbs. These tubes come in all sizes, from a 3" 5 watt bulb to much larger bulbs that replace 40W four foot (120 cm) tubes, yet are just one third of the size. The phosphor chemistry is what makes the difference between a cool white and a daylight tube and every tube is available with a dizzying array of choices in this area. Some of the most useful tubes for aquarists with small tanks are the 5000K compact fluorescent tubes. Fluorescent tubes are available in HO (High Output) or VHO (Very High Output) which draw more and much more current respectively, but produce more light than regular T12 tubes. As the composition of the phosphor changes so does the spectra of the visible light being emitted by the tube. For aquarium use, whether for illumination for plant growth or to simply be able to see inside the tank, only a small percentage of the dozens of available tubes are appropriate. They fall into the following broad categories: industrial, full spectrum, daylight, plant growth, actinic, tri-phosphor, special purpose and HO/VHO. Some types meant for aquarium use are shown in photo 2. In the photo below four Actinic and three "full spectrum" tubes are shown. You can see that they are marked in inches although sold in Europe.

To all appearances, the tube will put out the same amount of light until it suddenly stops dead one day, (which can take years), but for all practical purposes, because the drop off in light output is an exponential decay, the tube should optimally be replaced every six months or at the very least once a year. Writing the installation date on the tube itself with a permanent magic marker can be a big help here. Although fluorescent tubes come in many sizes, volume of scale dictates that there is really only one size - the T12 four-foot (120 cm) length. Nearly ninety percent of all fluorescent tubes made are this size, and because of this volume, this is the cheapest size.

Full spectrum tubes imitate, as closely as possible, natural sunlight by emitting light in every spectral range. All the different colours of visible light and a very small amount of ultraviolet is emitted. All these tubes have an output spectrum that is similar to sunlight - about as close as modern chemistry can bring us. These tubes try to imitate equatorial sunlight at noon, which has a colour temperature of around 5000K. Noonday sunlight from northern climes has a larger amount of blue in the spectrum, as has a colour temperature of 7500 Kelvin. Among them, we should note the tri-phosphor tubes which emit in the three basic bands blue, green and yellow. Since the phosphor producing the red colour is the most expensive one, usually there is less red emitted from those lamps (see photos 3 and 4).

A tri-phosphor fluorescent tube emits a full spectrum. Note the reduced amount of red. The complementary use of an incandescent bulb would make the ideal combination. This lamp gives the end user enough information as to what kind of light should be expected.

Another "full spectrum" tri-phosphor tube (photo above). Note the differences in the spectrum (far less yellow in this one) and the colour temperature (9500 K as opposed to 18000 K of the other one).

Next come the Actinic (or Actinic blue) tubes. These tubes emit light only from the blue end of the spectrum and are used in marine set-ups to supply the blue that is missing from normal aquarium lighting but is required by marine algae, anemones and corals (see photo below).

Other exotic fluorescent lamps 

In addition to the white lamps, other interesting types of lamps include all sorts of real colours (red, green, blue, yellow), blacklight lamps (which create a moonlight effect), germicidal lamps in which there is no phosphor coating at all and a quartz tube to transmit short-wave UV light (e.g., EPROM erasers and PCB photoresist activation), sunlamps, plant lights and special purpose specific wavelength lamps such as reprography and copier lamps.

HO, VHO  

There are also High Output and Very High Output types of lamps that have a discharge current of 0.8 A and 1.5 A instead of the standard 0.3 A. HO and VHO lamps are used when high light output is desired but are being outmoded by HID lamps like metal halide. The advantage of such a set up is less wires and less space occupied by tubes in your tank canopy. If we reverse that, they allow you to have much more light in your tank by utilizing the same (often limited) space in your tank canopy. The main disadvantage of these lamps is the cost, coupled to a considerably reduced lifespan. If space is not really a problem and an unusually high light level is not required then you should really go for the normal fluorescent tubes and spend the extra money on some reflectors or high frequency ballasts.

HID LAMPS

HID or High Intensity Discharge are the big bright lamps you see in grocery stores, street lighting and industrial lighting. They can be very large and draw a lot of power. Indeed 2000 watt and 6000 watt lamps exist, however small ones, down to 70 watts are available. These lamps produce a lot of light output quite efficiently, however they can be quite expensive to install initially and may require a fan for cooling in the housing/reflector as they can produce phenomenal amounts of heat. These lamps are used by aquarists who need lots of light, such as marine reef tanks, of large (and deep) freshwater plant tanks.

HID lamps require a ballast, and almost every bulb requires it's own type of ballast. The ballasts are expensive and bulky and are not something you trot on down to the corner hardware store to pick up, although larger hardware stores may have some; they are usually reasonably priced. You'll have to go to a lighting supplier for most of them however.

There are three basic types of HID lamps: mercury vapour, sodium vapour and metal halide:

Mercury vapor: When you see a bright light illuminating some industrial building and it has a characteristic bluish cast - that's mercury vapor. Mercury vapor lamps have an output spectra that is almost entirely blue-white, with very little red. Worse, the spectra is not continuous, there are spectral peaks at certain wavelengths. These lamps, although not useless - there is no doubt very good results can be obtained with them - are equivalent to cool white fluorescents. Yes they work, but why bother going to this expense and trouble when other bulbs will yield much greater success?

One interesting variation on this theme is the self-ballasted bulb. These bulbs (around 250 watts) require no ballast; they just screw into a standard medium base (i.e. incandescent) fixture and they simply work. The downside is that these bulbs are not as efficient as regular mercury vapour lamps because they use the resistive properties of the large filaments as a ballast, and worse of all these bulbs are very expensive. Of course with mercury vapour lamps having a 10,000 hour lifespan the high cost of the bulb must be considered in view of the lack of expense for a ballast.

Sodium vapour lamps: These lamps come in two varieties, high pressure sodium and low pressure sodium, although this is rather a moot point, as the light they output is monochromatic (pure) yellow, and is all but useless in terms of aquaria. It's rather a shame, as they are a full ten times more efficient then incandescent bulbs, in fact these are the most efficient bulbs made, and have a 24,000+ hour lifespan. These are one of the cheapest HID bulbs to Recent advances in high pressure sodium bulbs have improved output spectra, and are quite popular for terrestrial plants, although they haven't as yet gained great acceptance with aquatic gardeners.

Metal Halide:  Like sodium vapour, these lamps come in two versions, regular and colour corrected (HQI) versions. The HQI versions have a uniform, sunlight like output spectra, whereas the standard halide bulb has a lot of yellow, some blue and not much red. Unlike sodium vapour, these lamps are very useful to the aquarist needing a lot of light. They can be found nominally in 250, 400, and 1000 watt sizes, from most manufacturers, while some of them also make a 70 watt and a 150 watt size.

Duration of light

When keeping tropical fish, the duration of the lighting period should be between 10 and 12 hours daily. This should be a continuous period and you should never split it into two separate lighting periods. Special timers will enable you to apply a regular day / night cycle, by turning the lights on at the same time every day, even when you are not at home. An irregular lighting period will have a negative effect on your plant's growth and the well being of your fish. The fish need to rest and they have an internal "clock" which needs to be set. The factor that sets this clock is light. If the fishes can't get a normal day / night cycle they are going to be under stress until they do so.

It should be noted that, for species that do not come from the tropics (such as Mediterranean marine species) the duration of the lighting period should be limited to 7-10 hours daily while a seasonal adaptation would be desired. This means that a combination of winter settings (up to18^oC temperature / 7 hours lighting period) and summer (up to 26^oC / 10 hour lighting period) would be welcomed by your fishes (even more so by the other animals / plants living in such a tank). Bear in mind that the temperature at the surface of the water can be higher than at a deeper level. Thus, in a tank one meter deep one may observe a difference of 4 - 6^oC. It is advisable to take the measurement at the middle of the water column and definitely do not stick those "thermo - stickers" on the glass at the level of the gravel.

Light intensity

This largely depends on a) what we keep in our tanks and (consequently) b) how deep our tank is. Thus, deep water species require less light than shallow water species and the duration of the lighting period should be arranged accordingly. In the lake, the morning and afternoon sunlight does not reach the deep water which remains largely dark. Thus a 10-hour lighting period is more "natural" to them. On top of that, because of light scattering and absorption, only the blue rays reach this depth even during mid-day. Therefore the use of actinic lamps is recommended. A good combination is two actinic lamps coupled to a full spectrum white one (5500 K or thereabout).

For reef coral tanks, a very intense lighting is required. On top of that, reef tanks are usually deep (90 cm in depth or more, as opposed to freshwater tanks which usually measure 50-75 cm in depth). In this case, fluorescent tubes won't help much, unless you use too many of them. Using many of them may sound rational if you only consider the price of the fluorescent tubes. However, when you take into account the cost of the ballasts and the waterproof end caps, then other options may seem more appealing. For such tanks the much higher output required can be obtained by using metal halide lamps. Usually, a combination of metal halides with actinic fluorescent tubes is a very good choice.

For freshwater planted tanks, a bright white light is required. The fluorescent tube should be a good quality one, with a CRI (see glossary) as close to 100 (natural) as possible. Plants need both red and blue rays (green rays are also needed but to a lesser degree). For those tanks, one can use the special, high quality tri-phosphor fluorescent tubes. Sometimes you can use a combination of aquarium white tubes (5000-6500 K) with the normal plant tubes meant for terrestrial plants (the ones with an orange / red colour).

Most of us have been nourished in this hobby with the "so many watts of light / gallon or litre of water" rule of thumb. Of course this is an oversimplified rule and can easily lead to disasters. Firstly, it should read "watts of correct light". No coral will survive no matter how many household fluorescent tubes you add in your tank. Secondly, watts per litre mean different things to tanks of different depths. Thus, a lamp may serve efficiently a 20 cm deep tank while five of the same lamps will not serve a tank 1 meter deep. If we have two tanks with all the other dimensions being the same and different depths then an 1 meter tank will contain five times more water than a 20 cm one. However, the light which will reach the bottom of the deep tank (if lighted by the same light source) is only 1/25^th of that reaching the shallow tank!

Increasing the light intensity

Of course we are trying to increase the light actually reaching the fish, plants and gravel in our tank since there is (usually) no way to increase the intensity emitted by the light bulb we use. To do so, we usually rely on reflectors. There are two kind of reflectors: one is in the tube itself while the other is a light reflecting construction which fits in the tank canopy while the lamp fits in it. This has the advantage (especially during winter months) to reflect some of the heat produced by the bulbs in the water. Of course, this becomes a disadvantage during hot summers. A cheap equivalent is the construction of aluminum foil stripes, which are placed over the tubes, the shinning side facing the tubes of course. This can greatly increase the light entering the tank and costs nothing. On top of that, they are easily removed during the summer months.

It should be noted that more light will pass through clear water than through water with debris or other particulates. This is because the light will be scattered and reflected when it falls on those particles thus following random directions instead of reaching the bottom (or your plants). You should note that the presence of floating material (even air bubbles) will scatter the light unevenly thus shifting the colour balance - unpredictably. This is easily observed in tanks illuminated by actinic lamps only. The presence of tiny air bubbles or food particles, create the impression of "milky" water. In short, water, as a medium (and especially salt water) has a much higher refractive index than air. Thus, while air has a refractive index of almost 1, water has 1.33 (even higher in turbid water). Simply put, the higher the refractive index the more light is lost while travelling through that medium.

Ideally, the three sides of the tank and the cover should be covered with mirrors or reflectors, so no light would escape from the tank but this would induce too much stress to the fish. However, painting the tank sides with a pale light colour will increase the light that stays in the tank.

Special electronic ballasts can reduce the flickering of the fluorescent tubes and are highly recommended since they prolong the useful lifespan of the bulbs and consume less electricity (which is also an issue!).

Special tricks in Lighting an Aquarium

The correct lighting of the aquarium has two parameters, which the hobbyist must take care of. First, make sure the correct lighting setup for this aquarium is installed and working properly. The second is that this set up is not visible, light is not escaping from the canopy joints or covers and the wiring is neatly arranged (this has always been my weak point). After experimenting a lot with the lighting conditions in my tanks I have ended up with hundreds of meters of wires running along the tank, tens of starters and ballasts, a tremendous amount of heat when all the lights are turned on, and a terrible mess under my tank. This is because when I started my tanks I thought 4 tubes would be enough, so I made provisions for 4 sets of wires. Then came another four to increase the light reaching the sand bottom. Then came the four actinic tubes (while the first four lamps were never used again but stayed in place). Then came two black light bulbs for the "moonlight" effect and then, I decided to add some 9500 K tubes... In short, 24 tubes have found their way to my tank while I can't find my way to any of those tubes when I need to change it. So, plan first, execute later! If you plan to use many lamps, it could be a wise move to identify them. Thus, you can make a mark on the tube itself with a permanent marker and then make the same mark on the ballast and starter. This will make your life easier if you have to change or check a particular tube. If your tank design allows for that, put the starters and ballasts in a well aerated place and definitely not close to the intake of your air pumps. Blowing hot air in the tank is a plus in winter but becomes a minus in summer.

A specific colour of your fish or plants will be emphasized by adding one or more bulbs biased for this colour among the full spectrum white ones. Actinic blue lamps give a more natural appearance in many tank set ups, especially those with fish from deep waters.

Lighting your tank at an angle may reveal colours that are not visible (or so prominent) when the light comes from directly over them. This is especially true for most Malawi Haps which have much more intense colours when lighted at an angle of 60 degrees (light falling through the front glass on them). Those lights can be turned on while you are watching your tank and need not stay on during the whole lighting period.

You can create the effect of a sunrise / sunset easily with the use of timers. Ideally, you should use actinic tubes for a start, then the first set of white bulbs come on and after an hour or so the rest of the white lamps. For the sunset you simply reverse the order. There are sophisticated dimmers / starters available which will not allow the tube to reach it maximum lighting capacity immediately but allow for a gradual increase in light. A bit more expensive than the normal timer approach but it is really impressive and reduces the fish stress a lot.

Maintenance

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 vapour lights are unsuitable for lighting reef aquaria. Also avoid the HQL and HQI - NDL lights as their spectra and colour 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 colour 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.

 

 

 

Glossary

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 infra red (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 infra red 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.

Watts: 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.

Lumens: 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).

Lux: 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 (colour Rendering Index): The colour rendering index identifies the degree of colour 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 colour 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 colour 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 colour): 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.

Spectrum: This describes the wavelengths of light that make up the light source. Visible light (see Glossary) is a continuous band of colours 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: 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: 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.

References

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

 

Acknowledgements

Many thanks to Andreas Iliopoulos for critically reviewing this article. His thoughtful remarks added a lot to its integrity.

 

 

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