A Metal Halide Lamp is a high-intensity discharge light source in which the light is produced by the radiation from mercury, plus halides of metals such as sodium, scandium, indium, and dysprosium. The K at the end of color rating for each bulb stands for Kelvin. This is a measurement of the color temperature. The higher the Kelvin rating, the cooler (bluer) the color of light given off by the bulb. In general, the color given off by the 10,000K German is interpreted as being a perfect white color.
There are two main considerations to decide upon when chosing Metal Halide bulbs and systems: wattage and color temperature. There are generally two considerations when planning wattage, tank depth and the requirements of various types of corals. Tanks 18" and shorter will, typically, use 175 or 250 watt MH bulbs. Typically, tanks 24" and over will use 400 watt MH bulbs. I have found most soft and large-polyped-stony corals to do just fine within 5-15 inches from a 175 watt bulb and 10-24 inches from a 400 watt bulb. Most small-polyped-stony corals survive within 5-10 inches from a 175 watt but appear to thrive within 5-15 inches of 250 and 400 watt bulbs. These are all just general statements to assist in going the correct route with chosing wattage. They are not absolute truths.
Color temperature is a hotly debated topic where there is no one recommendation that will fit everybody. Lowers Kelvin temperatures are yellower and greener colors while higher Kelvin temperatures are whiter and bluer colors. There are two factors that determine most hobbysts choice of bulb colors, par rating and aesthetics. The 6500K Iwasaki bulb is generally regarded as an excellent PAR rated bulb. However, it`s green tinged yellowish color makes for what is considered by many as poor aesthetics. This is not the say the much bluer 20,000K German bulb is aesthetically ideal. Many hobbyists complain this bulb is too blue. By far, the bulbs most popularly purchased are the 10,000K German and the 12,000K Sunburst bulbs. The majority of hobbyists choose aesthetics as the determining factor of chosing color temperature for the primary reason that the appearance of our aquariums is of utmost importance. Due to the subjective nature of color preference, we refrain from making suggestions on which color temperature is right for you. Instead, we have a visual bulb comparison of the popular bulbs on the market.
As a general rule of thumb, use one metal halide bulb for 24" of tank length. Most of the Metal Halide bulbs in the same wattage format can be interchanged. However, please contact us to make sure. Metal Halide bulbs are one the most difficult aspects of the hobby to recommend. Most of the information here is general.
Common questions/problems concerning metal halide lamps: Metal halide lamps of the same kind are different colors?
Metal halide lamps operate by passing an electric arc through an arc tube that excites particles in the arc tube. When the metal particles reach an excited state they emit energy in the form of visible light. It takes 100 hours of operation before the metal particles stabilize and become a consistent color. All metal halides require 100 hours of operation before color and light output stabilize. All metal halides inherently have a slight color variation between individual lamps.
The metal halide lamp fires and then either burns dull or the arc extinguishes?
Usually the ballast and lamp are not receiving enough power. Metal halides pull a high current. Usually the problem is either there is too great a load on the circuit or the extension cords are of a small wire size and can`t handle the power needed by the ballast. Use only heavy duty extension cords and try running a ballast off a circuit from another part of the house.
Do I need a cover shield between the lamp and the aquarium?
Metal halide lamps produce a large amount of UV and also have a potential of exploding. The single ended metal halide lamps have an outer jacket which eliminates most of the UV. However, they have an inherent risk of exploding and UL requires metal halide lamps to be enclosed. On double ended lamps there is no UV shield. Double ended lamps must have a glass shield or major eye injuries including blindness may occur. Can I operate a ballast and lamp together which are designed for different wattages?
Each wattage lamp and ballast are designed together for optimal performance and safety. If you over drive the lamp it may explode and shorten the life of the ballast and lamps. If you under drive the lamp it will also shorten the life of both components and also may explode. Do not mix different type wattages of lamps and ballasts.
NOTE: Double ended bulbs should always be used with the UV shield (supplied). Your corals can suffer from excess UV radiation otherwise. The bulb should be handled carefully as well since it has no outer glass casing. Using soft gloves is a good idea to keep finger prints off bulb. This is during installation and replacement.
Bulb Replacement Guide
Bulb TypesLife of Bulb*
Normal Output Fluorescent6-12 months
Very High Output (VHO)6-12 months
Power Compact (PC)9-12 months
T5 High Output (T5HO)9-18 months
Metal Halide9-12 months
LED (non-moonlight)~5 years (50,000 hours)
* Manufacturer Recommendations
Since the North American operations began in 1967, it has been USHIO America, Inc.`s charter to provide customers with unrivaled customer service. With over 4,000 products in their portfolio, from unique specialty light sources for critically important and demanding applications, to general illumination lamps for architectural and task lighting, USHIO has earned worldwide respect as a company that delivers on its promise to meet the world`s lighting needs.