Most of the specimens kept in a reef tank, other than fish or mobile invertebrates, are photosynthetic.  That is, they receive part or all of their nutritional requirements from the lighting that they receive.  This is accomplished by the fact that they have symbiotic algae which lives within the tissues of their bodies.  The algae utilize the light for photosynthesis and the coral benefits from the byproducts of this process. If insufficient lighting is provided, the algae will die or slow down their activities and these animals will slowly starve to death.  Also, some of the colors of photosynthetic corals and clams are attributed to the symbiotic algae and the coloration a specimen will often increase under higher intensity lighting. 

Lighting is probably the biggest decision in setting up a reef tank. You can skimp on virtually everything else, but not this one unless you are willing to limit the inhabitants of your reef to those which can tolerate the lower quality light that you can provide. Like the size of your tank, your budget may determine for you the quality of light that you can give your reef. You cannot just walk into a pet store and buy that nice looking aquarium and expect the fluorescent hood that comes with it to work for your reef tank because it won’t.

 Lighting has two important characteristics that you must consider, intensity and color. Intensity is the brightness of the light and relates to how much energy the lighting is putting into the tank environment. Color of the light is also important as the photosynthetic processes that are occurring in the tank are targeted to work optimally with the color of the light normally available on the reef. 

Lighting Intensity

Intensity of the light relates to how much light energy is present at the surface of the water. There are a number of ways of calculating the amount of light you have or need in your tank, which are described below.

 One of the more popular units of measure being used in literature now is the PAR (Photosynthetically Available Radiation). PAR relates to the amount of energy provided in the spectrum that is useful for photosynthesis.  Unfortunately, there is no easy way for the hobbyist to directly measure PAR.  This rating is most commonly used to compare the efficiency of one lamp to another.

 The best way to actually measure the amount of light is to use a light meter made for this purpose. This type of meter measures lighting intensity in a unit of measure called Lux. A light meter will cost around $100 dollars. In general, the goal is to achieve a minimum of approximately 25,000 Lux at the surface of the water if keeping SPS corals with 20,000 to 40,000 being typical.  Lower light tanks can get away with less.  Measuring the light intensity using a light meter gives a completely accurate indication of how much light is really reaching your tank, however it is not really necessary.

 One way to ballpark your lighting requirement is to use a watts-per-gallon calculation. This method is not as popular as it use to be since it does not take some variables into consideration such as tank depth or the efficiency of the light source, but it is still useful. You obtain this number by dividing the watts of lighting in your light fixture by the gallons of capacity in your tank. For a high light intensity tank, you should use a figure of 8-10 watts/gallon as your basic target. A lower light tank can get by with approximately 3-6 watts/gallon.. How does this compare with the typical lighting supplied by a tank manufacturer? As an example, a 55 gallon tank with one 4-foot long 40 watt fluorescent light which is a typical store bought configuration would have a watts per gallon of 40W x 1 bulbs = 40W total / 55 gallons = 0.75 watts per gallon! Now consider that you should aim for approximately 8 watts per gallon in your reef tank. For this 55 gallon tank, you would need about 55 gallon x 8 watts/gallon = 440W! That is about 11 times the amount of light provided by the store bought hood! Since it is physically impossible to put 11 40watt tubes above a 55 gal tank, it should be obvious that we need to find more efficient lighting schemes for our tank.  If a low light tank is the target, you can see that the absolute minimum to achieve 3 watts/gallon would be 40W x 4 bulbs = 160W total / 55 gal = approximately 3 watts per gallon.

 A better approach, especially when dealing with Metal Halide lamps is to size the lamps by the depth of the tank and determine the number of lamps by the square footage of the tank surface.  

175W MH is suitable for up to 18” deep tank

250W MH for 18” to 24” deep

400W MH for anything deeper than about 24”

These are ballpark estimates only for a moderate intensity tank.  High intensity lighting might make use of 400 watt MH on tanks as shallow as 18".

 The other factor is the number of lights and the rule of thumb is that a MH lamp will illuminate approximately a 2’ x 2’ area of the tank.  A 4-foot long tank would require 2 MH lamps.  This is true whether the tank is 18” wide or 24” wide.  A 6 foot long tank would require 3 lamps and an 8 foot long tank would require 4 lamps.  If the tank is a more square configuration, the same basic rules apply.  A 5-foot long tank that is 36 wide has 15 sq./ft. of surface area.  Since each lamp can cover approximately 4 sq./ft., 4 lamps would be the appropriate number to use.

 Other major factors that affect the intensity of the light are:

The distance of the light from the surface of the water. The light drops off by the square root of the distance. Lights 4" above the water provide significantly more light into the water than lights 16" above the water. The only things that prevent the lights from being mounted with minimal spacing is heat buildup in the water, possible damage to the lights from water splashes and possible damage to the tank itself if it is made of acrylic or if it has a plastic brace near the lighting that can be damaged from the heat. Oh, and possible damage to your arm when you try to service the tank.

 Lighting Color

The color of the lighting must simulate the sun at some depth in the ocean. Sunlight in shallow water has not been affected much, but as the water deepens, the red and yellows of the light are absorbed while the blues and violet colors stay relatively strong. At 15 feet down on a reef, the light is tinted slightly blue. At 50 feet, the light is strongly blue and the overall intensity is lower.

 Color of lighting is typically referred to using the Degrees Kelvin scale. The lower the degrees Kelvin, the more yellow/red the color will be. This is also referred to as a low color temperature. The higher the degrees Kelvin, the bluer the color will be and is referred to as a high color temperature.

 We can relate this to our tank in the following manner.  The lower color temperature lights simulate a shallow reef, while a higher color temperature light simulates a reef in deeper water. Lighting on the market typically starts out at about 5500 °K.  There are even some being sold that have a 4300 °K color temperature. These have more yellow caste to them most people prefer. I recommend that a minimum of a 6500 °K light be used to simulate a shallow reef. These are often supplemented with blue lights called Actinic lights to give a slightly higher color temperature (bluer color). These actinic lights are also useful for simulating low light sunrise and sunset conditions in your reef. Other lights have a rating of 10,000 °K which give a crisp, bluish/white light which simulates a depth of about 15 feet. There are other lights that have a rating of 12,000 to 20,000 K which give a deeper blue light which simulates a fairly deep reef.

 In general, there is a tradeoff between the color temperature of the lamp and the intensity of the light it produces. The rule is, the higher the color temperature of the lamp, the lower the intensity of its light output. A 250W 6500K lamp will tend to have more light intensity than a 250W 10K lamp for instance.  10K lamps are the optimal compromise between the look and intensity for many reef tanks.  12K and above are gaining popularity, but frequently have a strong enough blue tint to them that some people do not like them.  The intensity is also pretty low.

 Another consideration when choosing a metal halide bulb is the CRI (Color Rendition Index)  The CRI index relates to how well the light reproduces the true colors of an object. This number ranges from 0-100 with the natural sunlight registering 100.  Artificial lighting rarely exceeds 95 and is often lower.  Most 10K lamps have a CRI of around 95 and thus do a better job of reproducing the true colors of the coral and other specimens than most other lighting.

Types of lighting useful for a reef tank fall into three primary categories, fluorescent, power compact and Metal Halide.

 Fluorescent Lights

Fluorescent lighting is the choice of many. While it is possible to be moderately successful with normal out (NO) lamps such as you buy at Home Depot, it is not possible to build more than a low light tank with these, even if the entire surface of the tank is covered with bulbs.  A much better choice is to use VHO (Very High Output) bulbs.  These VHO bulbs must be run on a special VHO ballast, but they provide about 3 times the light intensity of the NO bulbs

.Typical lamp wattages are shown below:

Let’s use our 55 gal tank as an example. Two 4-ft VHO lamps put out 220W which gives us about 4 watts/gal. This is sufficient for a low light tank. Four 4-ft VHO lamps put out 440W and gives us about 8 watts/gal which is enough for a high light tank. Four NO bulbs over the same tank will provide about 160W or 3 watts/gal which is a marginal level even for a low light tank. 

Actinic bulbs which have a strong blue color are almost always mixed with daylight bulbs in VHO setups in about a 50-50 mix.

 Power Compact Lights

PC lights are really just another form of fluorescent light. The bulb is essentially a skinny fluorescent tube that has been bent into a U-shape with both connections on one end of the bulb.  They tend to have a higher light output for the same wattage of bulb than the equivalent VHO bulb. PC lights are also available in small sizes making them a good choice for very small tanks.  Common wattages available include 9W, 13W, 55W and 96W.

 Some of the PC bulbs on the market have had poor reliability, perhaps because they are supported on only one end.

 The new incandescent bulb replacement tubes at the local Home Dept are forms of PC.  Their color is pretty yellow, but they have some use for growing macro algae or non-cosmetic lighting of tanks.

 Metal Halide Lights

Metal halide (MH) lights are the big guns in the world of reef keeping. Optimum lighting conditions for a high light reef is best achieved with MH lighting, especially if the reef is to be an SPS/Clam tank. MH lighting comes in many sizes from 70W to 400W and even higher. They are small compared to fluorescent lights and allow for a higher density of lighting over the reef. They are a pinpoint source of light, which gives them better punch (penetration) in deeper tanks. Being a pinpoint source of light also means that they simulate the effect of water ripple shadows similar to a natural reef, which most people find very enjoyable. Metal Halide is generally the optimum way to light a reef tank although you can frequently get away with less intense lighting systems depending on the livestock which are kept.

MH Types

There are primarily 3 types of MH lighting. Single-ended MH bulbs, Double-ended MH bulbs (HQI) and Mercury Vapor (MV). MV is technically not the same technology as MH, but some of the newer bulbs such as the Iwasaki’s have performance levels which put them into the same ballpark as normal MH.

Single-Ended MH
These are the most common MH lighting available. They use a large light bulb looking socket which they screw into called a Mogul socket. These bulbs come with an outer glass envelope which blocks the large amounts of UV radiation that these bulbs produce.

Double-Ended MH
HQI lamps require a double-ended socket for mounting. These bulbs do not have UV shielding and this needs to be provided as part of their mounting, otherwise the UV can burn the corals. These lamps tend to have some of the best color and efficiency performance, but also tend to be a little more trouble to deal with.

 Mercury Vapor
Mercury Vapor lamps are usually a very yellowish color. There is now a version made by Iwasaki that produces a fairly good white color. It is rated at 6500K color temperature, but seem to be very variable with some bulbs appearing very yellow and some being a very nice white. These bulbs are becoming fairly popular since they put out a higher level of PAR relative to normal MH. Some people find the color a little too yellow, but this can be minimized by supplementing with Actinic blue lighting. These bulbs tend to be cheaper as well.

Most MH installations also include Actinic fluorescent lamps as well in order to provide dawn and dusk lighting conditions. 

Metal Halide Ballasts

Metal halide lamps require a ballast to drive them.  The ballast provides the high voltage needed to ignite the lamp and the drive voltage required to keep the lamp running. The ballast is remotely mounted from the lamps due to weight and heating concerns.  There are two primary types of ballasts available.  One is called a TAR ballast.  These are low tech, heavy, large, low efficiency, low cost, but reliable devices and then there are Electronic ballasts that are high tech, relatively light and small, higher efficiency, higher cost and sometimes failure prone.  The ballast you chose will be determined by the type of lamp you are planning to run (Single-ended, HQI or MV) and whether you prefer the low tech (TAR) or high tech (electronic) approach to ballast construction.

 A popular TAR ballast on the market is the PFO ballast.  It has the benefit that you can get two ballasts in one housing, which is convenient (but bulky and heavy).  Advance is another common TAR ballast.

 Popular electronic ballasts include Ice Cap, E-Ballast and Sun Seeker.

Lighting Costs

Lighting costs can vary widely depending on how much work you are willing to do yourself and where you buy the parts. Prices below are ballpark MO pricing.

For our hypothetical 55 gal tanks, a four bulb VHO lamp system will cost approximately $175 for the ballast, $100 or so for four bulbs, $50 for holders and electrical connectors and some more money for reflectors. PC lighting cost would be similar.

For an equivalent MH lighting system, I would chose a dual 175W ballast ($175), two 175 10K lamps ($200) and reflectors ($60). The metal halide system will tend to be a little more expensive. Note that the wattage of the two 175W MH lamps is 350 watts total. This seems like less than the 440W VHO, but MH is a more efficient technology than VHO and you will actually get more usable light from the MH.

 Lighting Related Heat Concerns

Lighting systems can sometimes generate enough heat to cause excessive temperatures in your tank. The more wattage of light you have over your tank, the more heat it is producing. A misperception about MH lighting is that it generates more heat than fluorescents. Actually, MH lighting is more efficient and hence generates less heat for the equivalent light output. Since this heat is concentrated in a small area instead of over a long bulb, the MH bulb itself does get hotter and can be a serious burn hazard if contacted.

Heating from the lighting system can be managed via fans in the hood which cool blow air across the water surface and other fans which extract the hot air out of the hood. You do not want to blow the air directly at the lamps as that is now what you are trying to cool and excessively cooling MH lighting can shorten its lifespan and decrease its light output. In worse case scenarios, an external water chiller may be required to keep tank temperatures down.  

Further Reading:

Shedding Light On The Reef 
By R. Harker

Photosynthesis and Photoadaptation 
By S. Joshi

Spectral Analysis of Metal Halide Lamps: Part I
400 watt lamps (new) 
Bu S. Joshi & D. Morgan

Spectral Analysis of Metal Halide Lamps: Part II
400 watt lamps (used) 
By S. Joshi & D. Morgan

Spectral Analysis of Metal Halide Lamps: Part III
250 watt lamps 
By S. Joshi & D. Morgan

Reflecting On Lighting 
By R. Harker

DIY Guide to Compact Fluorescents 
By S. Joshi