Lighting the Reef Tank: A Primer for Beginners

The subject of reef tank lighting is one of the most controversial subjects in the reef keeping hobby. Lighting is often a hotly debated topic on Reef Central, because there are people who have had success or failure with each different type of lighting system used in the hobby. The most important consideration for reef tank lighting lies in choosing a lighting system based on the requirements of the animals that we wish to keep. While many reef keepers are concerned with the appearance of their reef tank, their main concern should be the health of the light requiring animals that they are attempting to keep. As many corals and other animals have symbiotic zooxanthellae that utilize light energy, the lighting system must meet those animals' needs, both in terms of intensity and color spectrum. The definitions of intensity and color spectrum are beyond the scope of this article, but I will include some useful references at the end of this article.

The commonly used phrase, "watts per gallon," is a way of expressing the lighting necessary for a tank by the ratio of the power used to illuminate the tank divided by the tank size in gallons. Many hobbyists suggest that an adequate amount of light would vary between four watts and six watts per gallon for low light coral and as much as ten watts per gallon (or more) for light loving coral. While such a rule of thumb may initially sound like a good idea, it should only be used as a rough guideline and not as a strict rule. Obviously, such a loose rule is misleading, and aquarists need to consider more than just "watts per gallon." Specifically, they must take into account the height and overall size of the tank. A tank that is 36 inches tall will require more light intensity to reach corals on the bottom than it would with a tank that is only 24 inches tall. For example, all 120 gallon tanks would require 480 to 720 watts of light based on the "watts per gallon" guideline, regardless of their depth, so a 120 gallon tank that is two feet tall, by four feet wide, and two feet deep would seem to need the same lighting as a 120 gallon tank that is three feet tall, by two feet deep, and three feet wide. The second tank, however, is a foot taller. Since the intensity of light decreases as the square of the distance from the light increases, the amount of light needed is much greater if the same corals are to be kept on the bottom of each tank.

There are many different types of lighting systems available to the reef keeper. They can be simple fluorescent tube lights, newer compact fluorescent lights or metal halide lighting. All of these lighting systems have advantages and disadvantages, but each may be successfully used to light a reef tank.

Fluorescent Lighting

Fluorescent lighting has been successfully used by reef aquarists for years. Two common forms of standard fluorescent light bulbs, normal output bulbs and very high output, are used in the hobby today.

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Fluorescent tubes
(1 - Actinic & 1 - Daylight)

Normal Output

Normal Output, or "NO," bulbs refer to the amount of power (in watts) used to illuminate the fluorescent tube. A standard NO ballast used with a 48-inch tube will use 40 watts to produce the light from the bulb. The following table depicts the wattage output based upon the length of the NO fluorescent tubes.   


The main advantages of NO lights are that they are commonly available, the parts (such as ballasts) are inexpensive, and the heat output of the lamp is fairly low. The main disadvantage of this type of lighting is that they produce little in the way of intensity in comparison to other types of fluorescent lighting. Aquarium inhabitant needs may require many tubes be placed over the length of the tank. The increased number of ballasts required for such a set up, if not mounted remotely, will add to the weight of the lighting fixture, and the heat produced by the additional ballasts may be significant and necessitate additional cooling. Using an electronic fluorescent ballast with these tubes, instead of the standard tar and coil ballast, can save electricity, heat and weight, but the initial cost is greater. Additionally, with the use of some electronic ballasts, the bulb intensity may be increased over a traditional tar and coil ballast.

Perhaps the best application for NO tubes are on short (18 to 20 inches or less), low light reef tanks, but the choices of which coral to use with this arrangement are very limited. Tanks using this lighting system typically house low light requiring mushrooms, certain soft corals and possibly some LPS corals that would need to be placed high in the tank close to the light. I have personally kept tanks using NO fluorescent lighting with some limited degree of success. The tank consisted of soft corals and mushrooms requiring only minimal lighting. While the corals did survive, their rate of growth was slow.

Very High Output

The second form of standard fluorescent lighting is Very High Output (VHO) fluorescent lighting. These tubes require a special VHO ballast capable of handling the higher wattage requirements. Although they appear similar to NO tubes, their power usage and intensity is roughly three times greater. For example, a four-foot VHO tube will produce 110 watts of light in the same space as a 40 watt NO tube. The following table depicts the wattage output with the length of the VHO fluorescent tubes:   




With this increase in light output comes additional heat, which can be easily dissipated by using a simple box fan or two in the tank hood or fixture.

The main advantages of VHO lighting are the high light output and the even spread of light over the entire tank, when proper length bulbs are utilized. The disadvantages of this type of lighting are that the cost for the ballasts is higher than the cost for those used with NO tubes. The heat created by the VHO tubes is also higher than NO bulbs. With their higher light intensity, a greater variety of corals can be kept under VHO tubes, including most soft corals, most LPS corals and even many light-loving SPS corals. Such VHO lighting works well on tanks that are less than 24 inches tall. One of my most successful reef tanks utilized VHO lighting, and I was able to keep a great variety of corals including some SPS corals. The soft coral and LPS growth was amazing. The SPS corals did grow well, although they were placed in the upper third of the tank.

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Acropora under 440 watts of VHO light
75 gallon reef with VHO lighting

Fluorescent Ballast Options

There are two types of ballasts available for fluorescent tubes: 1) core and coil ballasts and 2) electronic ballasts. The common ballasts that come with the standard NO and VHO bulbs and strip lights are core and coil ballasts. They are somewhat heavy, produce some heat and use more electricity than electronic ballasts. The electronic ballast does initially cost more, however, the savings in electricity and minimal heat output is, generally, well worth the initial cost. Electronic ballasts weigh far less than the core and coil ballasts, which allows more versatility in placement options. One particular model of electronic ballast, made by IceCap,™ Inc., can power both NO, VHO and power compact tubes in various combinations with up to four bulbs. It is claimed that the longevity of the fluorescent tubes is better when using the electronic ballast over the core and coil type ballast, but many reef keepers replace the tubes every six months anyway due to a shift in spectrum as the bulb ages.

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Fluorescent electronic ballast

Fluorescent Tube Color Options

There are quite a few different color spectrum options available in fluorescent tubes, with the most common being full spectrum, actinic and 50/50 (which is a combination of both). The most popular fluorescent tube color spectrums are: the daylight, 6500 Kelvin bulb that is normally recommended for SPS or clam tanks when enough tubes are used to create the proper intensity. The next is the combination Actinic and daylight bulb (a 50/50), which is equivalent to a 10000-Kelvin bulb and is well suited for soft coral and LPS tanks. The last bulb, called Actinic, is often used in combination with other bulbs to give a fluorescent glowing appearance to green and blue corals. Even though these tubes are recommended by various bulb manufacturers for high light applications, you must ensure that the light system is producing enough intensity for the corals that you wish to keep.

Since the price of aquarium grade fluorescent tubes is very close for both NO and VHO, the combination of the electronic ballast along with VHO tubes is a better value over NO tubes based on the amount of light output per dollar.

Power Compacts

Power Compacts (PC), also known as Compact fluorescents, are one of the newer additions to reef tank lighting over the last few years. This type of lighting employs fluorescent technology similar to VHO but instead of being a long tube with a connecter on each end, it is folded in half, giving it a "U' shaped appearance, and is powered from only one end. It is suggested by various manufacturers, that a 55 watt power compact bulb produces as much light output as a 95 watt VHO tube.

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Power compact lighting

The main advantage of the power compact lighting is that it allows more wattage within a smaller space than other fluorescent tubes, hence the term, "power compact." Power compact lamps come in wattages ranging from nine watts to 96 watts. The disadvantage of power compacts is that there are not as many color temperatures offered as there are with VHO tubes, although improvements are continually being made in this area and bulbs with different color temperatures are coming into the market.

Although, I have not had extensive experience with power compacts, I have used 55 watt 6700 Kelvin bulbs over one of my tanks. The light was very bright, however, the fixture was very warm when compared to the VHO tubes that I used on that same tank afterwards. A muffin fan should remedy this problem and cool things off considerably.

Metal Halide

If the many posts on Reef Central and other message boards are any indication, metal halide lighting is considered by many reef keepers to be the ultimate lighting for a reef tank. There are many different wattages and color spectrum choices. When proper wattages are used, almost any light-loving coral may receive proper illumination. The main advantage of metal halide lighting is that the bulbs are very bright and are the best option available to reef aquarists intent on simulating natural sunlight. These bulbs are available in wattages ranging from 50 watts to over 1000 watts. Some of the more common wattages used in the reef keeping hobby include 175 watt, 250 watt and 400 watt bulbs. Metal halide bulbs come in a wide variety of shapes, wattages and colors. Along with all of these options, comes a great deal of confusion when choosing this type of lighting for a reef tank.

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400 watt metal halide bulb on top of a ballast

Even though metal halide lighting sounds like a reef keeper's panacea, they do have one major disadvantage. Metal halide lamps (and some ballasts) produce an incredible amount of heat. The heat of these bulbs will easily burn the skin off your arm even if only touched lightly; I have the scars to prove it. This heat is transferred into the reef tank water and the room in which the tank is located, unless fans or other forms of cooling/ventilation are employed.

Unlike fluorescent tubes, which diffuse light fairly even across the tank, metal halide lamps are more directional in nature. The most intense light is found in the area located directly below the lamp. As you move from directly below the lamp to the sides, front or rear, the amount of light decreases with distance. Because of this decrease in intensity, Metal halide bulb manufacturers recommend that one bulb be used for every two-foot square area of tank surface. As an example, this would require two bulbs over a four foot long by two foot wide tank in order to achieve proper coverage. Since this light is more directional than other types considered, this type of bulb is useful on taller tanks or tanks where a large amount of light is needed to satisfy light-loving animals such as clams and SPS corals.

There is no rule of thumb when it comes to metal halide wattages, but there are some general guidelines to follow when choosing what watt bulb to use. On tanks 20 inches or less in height, one 175-watt metal halide bulb per two square feet of tank surface area will satisfy most corals' needs, as well as those of clams. For tanks over 20 to 30 inches tall, 250-watt metal halide bulbs will suffice. On tanks over 30 inches tall the 400-watt versions are useful for providing the appropriate intensities at the tank bottom. Now here comes the twist. Many reef keepers use higher wattage bulbs on relatively short tanks. It is not uncommon to see reef tanks under two feet tall using 400-watt metal halide bulbs in clam and SPS coral dominated tanks. I use 400-watt metal halide bulbs on my 90 gallon, 24 inch tall mixed reef tank with great success. The extra wattage helps ensure that my SPS corals, some of which are placed on the bottom of the tank, receive enough light. One other thing to take into consideration is that you can have too much light for certain corals and a great many other animals. Some large polyp stony corals often do not take well to intense light and need to be placed so that they are either somewhat shaded or located near the bottom of the tank.

Along with different wattages, different color spectra are available for metal halide bulbs. As is true with all bulbs, their color temperature is measured in degrees Kelvin (K). In simple terms, the higher the Kelvin rating, the bluer the bulb will appear. Since we are trying to meet the photosynthetic requirements of our corals, we want to give them the proper color spectrum to ensure their health and growth.

*Light Source Kelvin
Candle Flame 1800
Standard Incandescent Bulb 2500-3000
Warm White 3000
Cool White 4100
Daylight 6500
Noon Sunlight 5500
Overcast Sky 7000
Blue Sky 10000-20000+

*Color temperatures below 5500 Kelvin are not commonly used in reef aquariums.

When it comes to metal halide color spectrum, there is no one general rule of thumb, but some general observations can be made. Many hobbyists use metal halide bulbs with a Kelvin rating of 6500, and these bulbs produce a warm yellow/white appearance that simulates the midday sun on a shallow reef. Bulbs of this color are well suited for high light demanding corals and clams. While hobbyists report great success using these bulbs, they often complain of their yellow appearance when used without supplemental actinic light. Another common bulb is the 10000-Kelvin bulb, which produces a white appearance with a slight hint of blue. Although many hobbyists have also had good results using these bulbs over most types of corals, some have noticed that coral growth rates are somewhat slower than those using 6500-Kelvin bulbs. Gaining much wider acceptance and use lately is the 20000-Kelvin bulb, often called a Radium, which has a blue colored appearance. These bulbs are better suited for deeper water soft corals and LPS, although many hobbyists have also had success using them over clams and SPS.

I have personally used all of the above Kelvin bulbs with the following results:

The 6500 Kevin bulbs have given me the greatest coral growth in SPS, LPS placed lower in the tank and even soft corals. The color of the 6500-Kelvin bulbs when supplemented with actinic VHO tubes produces a crisp white appearance. For those seeking the greatest growth rates from SPS corals, I would recommend this color bulb.

The 10000-Kelvin bulbs also achieve good growth rates, albeit slower than the 6500-Kelvin bulbs. Their appearance is white with a slight blue tint when used with actinic supplementation. Bulbs of this color have produced excellent growth with soft corals and LPS in my own tanks along with slower paced SPS growth. I would recommend this bulb for a mixed reef environment.

The 20000-Kelvin bulb is very blue and brings out all of the fluorescent pigments in many corals. While they are visually appealing, the growth rate of my SPS corals came to a complete standstill while using them. I feel that these bulbs are well suited for a tank that simulates a deeper reef environment with LPS and soft corals but from my own experience, I do not feel that they are the best choice for high light SPS and clams. One note about the 20000-Kelvin lamps: in order to get the best intensity and color from these bulbs, they require a special HQI ballast for them to be driven as intended. This ballast is similar to the standard metal halide ballast, but includes a special starter to fire the lamp.

Metal Halide Ballasts

Metal halide lights do require ballasts and as with the fluorescent bulbs, the common options are the core and coil ballast and the electronic ballast. Where reliability and performance are concerned, the electronic ballasts have received mixed reviews amongst reef keepers. While some hobbyists have had success with electronic ballasts, there have been issues with the ballasts under-powering bulbs, interfering with TV and radio reception and problems arising from less than perfect home wiring. The electronic ballast may also interfere with the popular X10 wireless controllers that many hobbyists use with their reef tanks. Also, the initial cost of the electronic ballast is higher than the core and coil ballast; however, this can possibly be compensated for in terms of its electricity cost savings over time.

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Dual 400 watt metal halide ballast
(Core and Coil)

The core and coil ballasts are very large, heavy and create a significant amount of heat. These ballasts also use a great deal of electricity. The advantage to this type of ballast is that they are normally very reliable and are typically less costly than their electronic counterparts.

Combination Lighting

Another common practice among reef hobbyists is to combine different forms of lighting to achieve the proper spectrum and intensity in the reef tank. Many hobbyists use metal halide bulbs along with Actinic fluorescent tubes to satisfy the requirements of the corals and obtain an appearance that is visually appealing. A common example is the use of 6500 Kelvin bulbs with Actinic fluorescent tubes. The Actinic tubes offset the yellowish appearance created by the Iwasaki bulbs and bring out a crisp white appearance. There are many other combinations possible and, unfortunately, it takes experimentation to find a combination that produces good coral health while still being aesthetically appealing.

In Conclusion

Unfortunately, there is not one particular lighting system that will ensure success for all corals that many hobbyists want to keep. As long as the hobbyist does his/her best to match the requirements of the corals with the type of lighting system, this will go a long way towards achieving success.

I would suggest finding other reef keepers in your area who have the type of lighting systems that interest you and go see their tanks with your own eyes. Look for the overall appearance of the tank, the health of the corals and the placement of corals in relation to the light. Also, ask many questions on message boards such as Reef Central, since many hobbyists have already gone down this road, and can give advice based on their experience using various lighting combinations. click here for full size picture click here for full size picture

If you have any questions about this article, please visit my author forum on Reef Central.

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Lighting the Reef Tank: A Primer for Beginners by Doug Wojtczak -