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. Please click here for more information on how the different Kelvin ratings appear.

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.