Lighting for Saltwater Aquariums


Most corals offered for sale come from reef flat and protected lagoons of leeward fringing reef biotopes. Because of this, many of the corals that we see at the local pet shop are from the shallower regions of our reefs where sunlight is very intense. Of course there are certain specimens that come from deeper reef areas as well as overhangs and caves where lighting is less important. When we decide to purchase coral for our tank, we must take into consideration the amount of light that we have for our tank as well as the type of coral we plan on keeping. This important equation needs to be addressed if we are to be successful with keeping various types of coral. Yes, there are other factors that play out in the overall evaluation and ability to grow coral for the long term. Variables such as pH, alkalinity, turbidity, temperature, salinity, trace element dosing, water flow, filtration, calcium level and the list goes on.

For now I will focus just on the lighting part of the equation. Before I get involved with the lighting and how much, color rendition, kinds of lighting fixtures and bulbs etc., I need to give a bit of an introduction on corals behavior and its importance to the light connection.

All corals require some sort of light intensity to survive, taken one steep further and symbiotic invertebrates and Tridacna clams also benefit from some form of light, again depending on the husbandry issues.

Photosynthesis and calcification rates in corals are enhanced by providing light in the blue and white area within the spectrum. Light wavelengths toward the red end of the spectrum may induce undesirable amounts of micro algae growth and is sometimes associated with bleaching. Chlorophyll is the primary photosynthetic pigment that gives the single celled zooxanthellae its brownish/cream color. The lighter the color, the shallower the water that the coral is from. Likewise, the darker colors are indicative of deeper water species.

There are different terms that, in essence, make up the varying lighting concepts. Listed below are many of the terms often associated with the right lighting make-up for your particular tank style.

INTENSITY-measure of how much light energy(photons) is available.

IRRADIANCE-the amount of light energy that strikes a predetermined area, over a period of time. This concept is often described as lux. Lumens can be measured in terms of lux. For example, one lux =one meter square.

LUMEN is the measure of the perceived power of light/brightness.

PAR(PHOTOSYNTHETICALLY AVAILABLE RADIATION) represents the available amount of radiation between 400 and 700nm(wavelength-light) that zooxanthellae could potentially use for photosynthesis.

PUR(PHOTOSYNTHETICALLY USABLE RADIATION)is the amount of radiation that according to pigment concentration(chloroplasts/chlorophyll) water clarity etc. that is actually usable coral zooxanthellae.

PSR(PHOTOSYNTHETICALLY STORED RADIATION)this is the ACTUAL amount of light harvested and converted into chemical energy required for photosynthesis.

CRI(color rendition index)is the term for measuring the spectral rating at a given time. A perception of the human eye as it see light. For example the suns CRI is normally 100. A Vita-Lite supreme fluorescent bulb offers the same spectral rating of 5500k, has a CRI of 96 and 2000 lumens, making it one of the closest matches to natural outdoor light.

SATURATION RATE is the maximum amount of light that a corals zooxanthellae population can heandle for photosynthesis. This is usually between 20,000-30,000 lux.Any more than this and corals will have to deal with the excessive irradiance metabolically, often times, resulting in bleaching due to an overabundance of oxygen in the corals tissue.

KELVIN-term used to describe the color temperature(degree kelvin) of the suns rays at various depths, or the color of our artificial bulbs used to light our tanks and provide irradiance for our corals zooxanthellae and the photosynthetic process.

Sunlight has a kelvin temperature of around 5500 to 6000kelvin(k). This color is characteristic of the red/orange/ yellow end of the spectrum and is know as a cooler temperature. At the other end of the spectrum comes the blue and purple end of the spectrum and is known as a warmer temperature light coloration. To put this in simpler form, the spectrum of light starts off with Infrared, red, orange, yellow, green, blue, purple and ultraviolet and from 400-700nm. Taking this one step further, looking at the coloration of the suns rays in respect to depth if water and what happens to the light spectrum as water depth increases, this is what happens:

At the waters surface, say at midday under a cloudless sky, and the irradiance is over 120,000lux. On average the irradiance at the waters surface is around 75,000lux. As light travels down 15 feet, the irradiance level drops to roughly 20,000lux. At 30 feet, the level drops again substantially to 10,000lux and so on. The color of the wavelength spectrum is also altered as water is a refractor of light. At just 15 feet in depth, the red and orange wavelengths of the sun have been filtered out, by the water as a result of these colors being refracted back into the earth’s atmosphere. At 30 feet, the yellow end of the spectrum is gone. At 50 feet and the green light is gone leaving just the blue and purple wavelengths, thus the reasoning that the oceans of our world look blue in color.

Now, obviously, our tanks aren’t this deep, which is why there are different bulb colors available to us for use on our tanks. To replicate a particular water depth environment as well as to add to eye appeal and coral health/ growth. Having given you a broad sense of lighting, its terminology and effects, I will now offer you some of the various lighting types, for practical purposes, available for fish only tanks up to and including a full blown reef tank, complete with corals, inverts and fish. Obviously, if you don’t plan on replicating a reef tank, lighting is not an important issue or required for the success of your livestock. However, if a reef is desirable, then much though will have to be placed on the type of coral you plan on keeping and the light they require for survival.

FLUORESCENT LIGHTING: Available in basically three different forms. VHO, normal output and power compact. These bulbs emit light over the entire length of the bulb. They require less electricity for the most part and are not as strong as halide bulbs. They come in a wide range of colors and sizes such as T5’s and normal fluorescent bulbs. The heat issue is also less evident with fluorescent bulbs as opposed to halides. Many fixtures contain fans built into the fixture to reduce and disperse the heat issue generated by any bulb, especially since the light duration is pretty extreme. Most corals and certain clams can be kept under these lighting schemes.

METAL HALIDES: These lights are very intense and generate a lot of heat…much more than your above mentioned bulbs. They are a “point source” bulb where the the light emitted is generated by a filament in a small bulb and not over the entire length of the tubular bulb found by fluorescents. The electricity involved with halides is usually more evident with halides than fluorescents.

Halide bulbs come as either a mogul style(screw in) in HQI DE(double ended snap- in). Overall,the DE bulbs are more intense than the mogul style and contain just one hot spot as opposed to that associated with mogul bulbs.

Metal halide bulbs also create strong glitter lines that, in essence, create ripples in your tank. These ripples can act like moving lenses increasing the light intensity to your corals below. Magnifying the available light up to 200% from 1-4 times per second will amplify the light energy to a corals zooxanthellae, and also offer an aesthetically pleasing look to your tank. Halide fixtures can be a mixture of both power compact or T5 bulbs with a halide bulb or solely a halide bulb. They can be either in pendant form or a lengthy light fixture.

Type of coral is only part of the equation in making your decision on the right lighting fixture for your tank. Size and depth of your tank should also play a part in your overall decision as to lighting arrangement.

Other variables can affect the amount of light that is available to your tank such as dissolved organic matter(DOM) like a yellowing tinge to your water known as (Gelbstoff). Turbidity, suspended particles, salt creep accumulation on your glass protective cover also need to be addressed in establishing the best and most efficient lighting scheme for the overall success of your inhabitants.

Hopefully, this will help you to make the best overall decision as to the ideal lighting for your tank and its inhabitants.

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