Just wanted to correct a slight error in description of Kelvin re Light temperature.
For example a 5500K bulb will appear more yellowish than a 10K bulb. The 10K bulb will appear more blue/white. Actinic bulbs dont have a kelvin rating because most of their output is beyond our visable range. This is why these bulbs often appear simular to black lights or have a very deep purple coloration.
Actinics do have a temerature rating as do any light generating objects. However, the means of determining that are different in flourscents as it is a chimical reaction of gas molecules at the atomic level that generate the light in these bulbs. Therefore, it is more difficult to determine the temp. rating. In addition, the temperature rating would have less meaning as these bulbs do not have high CRI's (color rendering indexes). This is because by combining various substances in the bulbs we are effectively filtering the lightbulbs output, i.e. tailoring it to have a very specific spectrum.
Note that the temperature of a bulb does not exactly relate to it's color spectrum, it actually relates to the peak wavelength of light that the bulb (or object) produces. The color spectrum can only be determined by a variety of other ratings and methods. Though it is generally true that the cooler the bulb the warmer the color and the hotter the bulb the whiter/bluer the light. However, the 10,000k and 20,000k bulbs peak in the Ultra-violet wavelegths, hence visibility of the light is not related to the temperature of the bulb(object).
This formula converts Temperature to Wavelengths
WaveLenght in Nanometers = 2,900,000/ temp in Kelvin
2,900,000/10,000 = 290 nM
Visable light ends at about 400 nM so a 10,000 K
bulb has it's peak output well outside the visible spectrum. A 10,000 K
bulb actually produces a considerable amount of ultraviolet light and this is likely the reason for it's popularity in Reef systems as it increases coral organism's generation of pigments (colors) just like it will make you or I tan.
Just thought I'd throw some additional light on the subject.