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        The Role of Artificial Lighting In A Reef Aquarium

Most of us learned in school that all  life relies on the existence of sunlight. Although it's true that the recent discovery of life supporting thermal vents, at the bottom of the ocean, have weakened the hard and fast rules a little, for our purposes they continue to hold true - our reef aquariums, and most of the life in them (with the possible exception of a very few invertebrates, such as non-photosynthetic gorgonians and the various Tubeastra and Dendronephthya species), depend on the availability of light. And not just any light. Most of the corals we keep in our reef tanks come from shallow seas located on, or at least fairly near, the equator. Anyone who has had the experience of visiting such a place knows that the intensity of sunlight in the tropics is very high. It is this enormous amount of light that provides the energy that drives photosynthesis.

The tissue of most corals kept in modern reef aquaria contain symbiotic algae called zooxanthellae. These algae require light to perform photosynthesis and in order to produce  the simple sugars that corals require as food. Corals and their symbiotic zoozanthellae  have spent millions of years adjusting to and optimizing their assimilation of the sunlight flooding down on them.

In order to be successful at keeping corals, especially the sensitive SPS (small polyp stony) corals, we have to provide a light that is intense enough and includes an acceptable color spectrum, and we have to provide it for an optimal period of time.

Light intensity and photoperiod

The amount of light we provide our reefs can be measured as a combination of light intensity and photoperiod. PAR (photosynthetically active radiation) is a currently popular buzzword. Indeed, it is an important means of quantifying the amount of light reaching our corals, but it is only a momentary  measurement. Light intensity in a "natural reef" at a depth of 12 feet can vary from 200 to 1000 PAR, depending on the time of day and the amount of cloud cover. This tells us that the corals we keep are definitely not adapted to receiving a steady light intensity.

Most experts agree that 400 PAR is an "advantageous" level of light intensity for most of our shallow reef SPS and LPS coral species. In the Glass Reef, PAR readings vary from approx. 700 at the water's surface to approx. 300 when measured just at sand level. So, if we say that these are optimal  levels of intensity, what happens if we provide this light intensity for 12 hours a day? Is that better than 10 hours a day, or would 6 hours be better?

The combination of light intensity and photoperiod is a subject that I have not seen discussed very often - and when it is discussed there doesn't seem to be much depth in the facts/research being presented. I know that there are scientific studies which show that, if the light intensity is adequate, the percentage level of mitosis (cell division) in water plants is dependent on the photoperiod being close to normal (the natural period). Optimal light intensity and photoperiod produced mitosis levels of approximately 50% - half of the cells were dividing at any one time.. It was shown that photoperiod, both below and above the natural level tended to decrease the average mitosis level. This means longer (or shorter) is not better, at least for water plants. I would think that there is probably no reason to assume that these findings cannot be used for zoozanthellae. This leads me to the conclusion that work is still required in determining the optimal photoperiods in our reef tanks.

Light spectrum

The wave lengths of the visible part of the light spectrum reaches from violet, approx. 380nm (nanometers), through green, yellow, orange, and finally red at approx. 780nm. Below the violet range (120nm to 380nm) we have the ultraviolet. Ultraviolet light has a large influence on our corals, as we'll discuss. The spectrum past red (infrared) is be generally considered as heat radiation.

Ultraviolet light is divided into three regions:

  • uvA - (315nm to 380nm) can pass through normal glass and is important for autotrophic organisms (organisms that produce complex organic compounds - carbohydrates, fats, and proteins - from inorganic molecules using energy from light ).

  • uvB - (280nm to 315nm) this is the medium bad stuff. It'll give you a sunburn and can damage any living tissue, including corals. It cannot pass through normal glass.

  • uvC - (100nm to 280nm) this is the really bad stuff. For the most part it is absorbed by the earth's atmosphere. The UV light units that are used by some (including me) emit UVC light. This is why they are so efficient at killing bacteria, algae spores, etc.

So, a little about "normal" light. Thousands of articles and millions of words have been written about corals and the "best" light spectrum. The consensus seems to be that blue is the most important "growth color". Why should this be? Well, it all has to do with seawater. As light from the sun enters the oceans, it is white - the combination of all colors - but seawater filters out certain colors. Red is the first part of the spectrum to go. At a depth of 12 feet, red has, already, mostly disappeared and ultraviolet light has been greatly diminished. The deeper we go the bluer the visible spectrum becomes.

Color Temperature - those plants that require light for photosynthesis (including our coral's zoozanthellae) have adapted themselves to the prominent color in the "underwater spectrum" - blue. It is for this reason that the industry that provides the hardware we employ to produce the light used for our reefs has concentrated on light sources that deliver a color temperatures of between 12000 degrees Kelvin and 20000 degrees Kelvin. What does that mean? Well, the color temperature of a light source describes the color of that source compared to an ideal black-body radiator (a theoretical object that  totally absorbs all thermal energy that falls on it and therefore appears black - as the energy is absorbed the object heats up and re-radiates the energy as electromagnetic radiation) that radiates light of comparable hue to that light source. The temperature value is normally stated in units of degrees kelvin [K].  Higher color temperatures (5,000 K or more) are called cool colors (bluish white); lower color temperatures (2,700–3,000 K) are called warm colors (yellowish white through red).

Artificial lighting

So, what we are looking for is a light source (or sources) that will deliver light that is sufficiently intense and radiates in the desired spectrum so that the zoozanthellae residing in our corals will have ideal conditions for photosynthesis. Sounds simple - well maybe. Knowing what lighting conditions must be met in order to theoretically guarantee the health of the corals in our keeping is just the first step. Decisions must be made about what light system to employ.

As far as types of light emitters (bulbs) are concerned, the last couple of years have greatly narrowed down the playing field:

  • metal halide  (MH) these are the big guns. Available in power up to 1000 watts and 6500 Kelvin to 20000 Kelvin color temperature.

  • fluorescent (T5) although power compact (CP) and VHO fluorescents are still available, the T5 bulbs have become the standard.

  • light emitting diode (LED) the latest emerging technology. I think they have a great future but, it may be too soon to make the jump.

In addition to light sources, the type of reflectors to be used must be decided upon. The importance of quality reflectors should not be underestimated. Experiments have shown that well designed reflectors can deliver twice the amount of light (and in some extreme cases, even more) than badly designed models. I would go so far as to say that, when it comes to lighting, any purchase should be made with quality in mind. A modern reef aquarium is not exactly an inexpensive investment, in both time and money. Lighting is one of the areas where you definitely don't want to sacrifice function and reliability for the sake of saving a dollar here or there.


It can't be denied that we all want our aquariums to look beautiful. The lights we employ shouldn't just promote coral growth, they should also enhance the viewer's visual pleasure. This can be accomplished by employing lights that deliver not only tank inhabitant "healthy" lighting but evidence a color temperature that is pleasing to the owner's taste. As an example, I find that when it comes to metal halide lighting, 20K bulbs produce a light that is too blue for my liking. MH bulbs rated at 12K or 14K are much better in that respect. They just look more "natural". When it comes to supplemental lighting, T5 bulbs for example, there are dozens of combinations possible. Depending on what bulbs are used, it is possible to really make colors in the tank "pop".



Lyfey says...
Great writeup!


GlassReef: Thanks, Chris. Actually, it's not finished yet. I've got quite lot more to add. It should get better. Cool
21st October 2010 3:27pm

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