When selecting a light bulb for your saltwater aquarium, you have most likely run into terms like 6,700°K, 10,000°K, 14,000°K, 20,000°K, Actinic, Daylight, 420nm and 460nm.
Kelvin Scale
The numbers 6,700°K, 10,000°K, 14,000°K, and 20,000°K make reference to the Kevin Scale. Lower Kelvin degrees, like 6,700°K are more yellow and higher Kelvin degrees, like 20,000°K are bluer. The most important factor when selecting a bulb is to consider its color and intensity. A 6,700°K bulb will give the aquarium a yellow hue and will promote faster coral growth than a 20,000°K bulb. Some aquarists prefer the blue look of a 20,000°K bulb as it brings out fluorescence in many corals. Most hobbyists find that a combination of 10,000°K and actinic bulbs will create strong coral growth, a crisp white look and coral fluorescence. Although finding a balance between daylight and actinic bulbs can be a challenge.
Daylight & Actinic Aquarium Light Bulbs
Daylight Lighting usually references bulb temperatures between 6,700°K and 14,000°K. Actinic Lighting refers to bulb temperatures between 14,000°K and 50,000°K. You may also see abbreviations like 14K or 20K. Actinic lighting has many different names. Each manufacture has their own set of terms, like super actinic, actinic plus, actinic blue, blue plus, etc. The key is to determine where a bulb’s color spectrum peaks on a wavelength chart. If the wavelength peak is not already specified, a spectrum chart is usually provided by the manufacture. For example a 420nm actinic bulb will have a deep purple hue and encourage coral fluorescence, whereas a 460nm actinic may be a brighter blue and only provide some coral fluorescence. Please note, a 10,000°K bulb made by manufacture A will have a different color than a 10,000°K bulb made by manufacture B.
Unless you are extremely concerned about coral growth, select a combination of bulbs that is visually appealing to your eyes. It may take a few tries to get it right, but it will be rewarding when your aquarium and corals look gorgeous. If you are using only fluorescent bulbs, most hobbyists prefer a 1:1 or 2:1 combination of Actinic to Daylight bulbs. Remember that higher Kelvin bulbs are bluer and have less intensity than a lower Kelvin bulb of the same wattage. Daylight bulbs are also more intense than actinic bulbs.