Metal halide lamps are now available in a wide variety of wattages as well as color temperatures. Wattage determines how much light is produced, with higher wattage bulbs producing more total light. However, the amount of light produced is not in a direct 1:1 ratio with the wattage. That is, a 1000-watt bulb does not produce four times as much light as a 250-watt bulb, it produces more light, but it is only a percentage increase. Color temperature refers to the spectrum of the bulb. The higher the color temperature of a bulb, in degrees Kelvin, the bluer the light that will be produced. That is, a lamp that is rated at 6500 degrees Kelvin like the Iwasaki 6500 bulb produces a light that appears greener relative to the light produced by a lamp rated at 20,000 degrees Kelvin such as that made by Radium. Many metal halides require a 100-200 hour burning in period until their true color comes out. So the color of a new bulb may be significantly different than the color of the bulb it replaced so this should be kept in mind when bulbs are replaced.

Initially it was thought that bluer lights, with higher Kelvin temperatures, would produce more useable light. That is, that the amount of total light that they were producing was equal to that of the lower temperature lights but that it contained a higher percentage of light with the PAR wavelengths. What has been found in some studies is that these bulbs do indeed produce a large percentage of their light in the blue spectrum, but they do this by producing less total light, particularly at both the red-orange and the green-yellow wavelengths. This may be important, as the main reason for using these bulbs is to get enhanced or vivid coloration in the SPS corals. This mat occur, but at the expense of more rapid growth. While the exact mechanism(s) for producing brightly colored SPS corals are only now beginning to be understood, it is known that only when strong lighting is provided will growth in these corals be strong enough to produce brightly colored tips. As a result high temperature bulbs may not necessarily provide the best light for optimal growth and coloration in all corals.

Another aspect of keeping a reef tank should not be neglected when considering bulbs: the pleasure of viewing the tank. While lower temperature (6500) bulbs may produce the most rapid growth and greatest amount of total light, the tank itself may appear stark and have a yellowish tint. In order for viewing of the tank to be as pleasurable as possible it usually is necessary to add some blue light. When these metal halides are used bulbs such as the PC Actinic bulb or the T5 Actinic blue bulb should be used in conjunction with them.

I have now experimented with a large number of bulbs as well as combinations of these bulbs. I have also seen several hundred tanks lighted successfully with virtually every combination of bulbs as well. It is my opinion that the best way to light a large SPS dominated tank at present is to use a combination of either 6500K bulbs or 10,000K and 20,000K bulbs with or without some additional Actinic fluorescent lamps. It should be noted that in order for this combination to maximize coral growth and coloration it may be necessary to replace the 20,000 bulbs approximately every 6 months as the intensity of these bulbs seems to drop off rapidly beyond this age. This is necessary as these bulbs not only produce a relatively lower temperature over time, but they also produce less light over time. As a result, they need to be replaced far more frequently than the 6500K or 10K bulbs, which do not need to be replaced until the 12-18 month mark. It has been my unfortunate experience that when lighting is inadequate corals fail. As a result I have learned that it is much cheaper to replace bulbs on a regular basis than it is to replace corals so I now replace the bulbs on an aggressive schedule as soon as their intensity diminishes by as little as 10%.

Another aspect of using metal halides that needs to be addressed is their ability to burn SPS corals if the corals are not gradually acclimated to them. Corals taken from less bright tanks or that are suddenly exposed to new bulbs can rapidly shed their zooanxthellae, which they can also do for a number of other reasons as well. To reduce the likelihood of this happening several precautions should be taken. Newly acquired corals should either be placed on the bottom of the tank and gradually moved up, or they should be screened from the light in some way. They should then gradually over 1-2 months be exposed to the full light. Similarly when lights are replaced or added, the photoperiod should be reduced and some means of reducing the initial intensity should be employed. This can be as simple as raising the lights up or placing some shade cloth or egg crate above the tank during the acclimation period. These little precautions can dramatically reduce the likelihood of losing corals due to burning.

Several other aspects of metal halide lamps need to be considered before the lamps are chosen. These lamps are generally expensive to purchase, expensive to run owing to the large amount of electricity they consume as well as the heat they produce. This last attribute should not be downplayed in that if a lot of these lamps are employed or if they are placed in a small or closed space it may be necessary to run a chiller or other means for removing the heat produced in order to keep the tank’s temperature within the desired range. This will further add to the expense of using this light and it should not be down played when considering between the choices. One possible way to reduce these problems is to use electronic ballasts such as those manufactured by Icecap. These ballasts typically run much cooler than the older tar ballasts and thus send less heat into the room in which they are located. They come in a variety of sizes and will power lamps ranging from 175 to 400 watts. While it is my opinion that metal halides are the choice in lighting for large SPS tanks I have also seen SPS corals kept very successfully when only VHO fluorescent lights were employed. These tanks were smaller and the corals were placed high up in the tanks directly under the lights. In some of these tanks, the SPS corals were at least as brightly colored as those in tanks using metal halides. A method that combines the best of both worlds is to use mostly fluorescent or power compacts to light much of the tank and then to also use pendants to provide a spotlight affect from one or two metal halide bulbs. Aluminum pendants such as those manufactured by Aqualine can be used to focus the halide light on one or two spots where sps or other animals requiring bright light are located. The use of these pendants in this manner can also introduce dramatic lighting effects by producing strongly lit areas along with more dimly lit ones. This can enhance the viewing pleasure of a tank.

For smaller systems or where soft corals are the predominant coral kept metal halide lighting may not be the best choice for lighting a reef tank. Fortunately, improvements in fluorescent lighting now make them a viable alternative in many instances. Up until a few years ago it was not possible to get enough fluorescent tubes above a limited space to produce the desired amount of light. However, with the advent of very high output (VHO) fluorescent tubes and more recently power compact (PC) fluorescents and the even newer T5 lights, it is now possible to get the amount of light desired over even a relatively small space. In addition, there are now a wider range of color temperatures available for these bulbs as well a variety of sizes and wattages. The goal for optimal coral health and growth is still to provide 4-5 watts of light per gallon. Regardless of which type of bulb is chosen they should be in a 1:1 ratio of Actinic Blue type (12,000 or 20,000K) to the Daylight (6500K) or Actinic White type (10,000K0). I have used this combination for over the past 15 years and have seen it used in a large number of tanks with a great deal of success. This combination can be used whether power compacts or VHO fluorescents are chosen, there does not seem to be a significant difference.

The choice of PCs or T5s or VHOs used to come down to price, but with the price of these lights becoming closer it now comes down to which light looks best, with PCs or T5s being the best choice when only soft corals are being kept. There are several reasons for this with the first being that for smaller tanks the PCs and T5s provide more light over small areas than can be achieved with VHO tubes. All of the fluorescents also offer another advantage in that there is little likelihood that these bulbs will burn the corals. As mentioned above these lights also seem to provide the intensity and spectrum to allow for both growth and coloration in SPS corals, so these bulbs should not be limited to only soft coral tanks. As mentioned their only drawback is that for large tanks the number of bulbs and ballasts required to light the tank make them cost and space prohibitive.

These lights do have another potential application and that is in combination with metal halides in order to get more blue light over the tank. In tanks where only 1 or 2 metal halides are being used it is usually desirable to add some actinic lamps to balance the harshness of the metal halides. These lamps can be used to simulate sunrise and sunset so that the animals are not startled as would be the case if the halides just came on. Plus thee bulbs also cause some corals to biofluoresce and show beautiful colors, which greatly enhances their attractiveness.

Lighting a reef tank properly is usually one of the most expensive components of setting up a reef tank, so a lot of time should be taken to make sure that it is done right. When it is done properly the health of the corals is ensured and the tank is visually appealing. When a tank is properly illuminated the corals will grow rapidly, the colors of the corals and fish will be as brilliant as when they are on the reef and the entire set up will be quite impressive. So when deciding on which lighting option to choose take your time and try to view tanks similar to the one you want to set up illuminated by different systems. This will allow you to do it right the first time and avoid the really expensive step of having to do it a second time.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.