Kelvin ratings of aquarium fluorescent bulbs var quite a bit. Here is what they mean and how they pertain to saltwater aquarium lighting.
5500K (Bottom end of the acceptable CRI)
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Reason: Frequencies below 5500K promote algae blooms as these algaes prefer less blue-white light and lean to the red-yellow absorption spectrum.
6500K (Middle of the road CRI)
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Reason: Best all-around starter frequency, often used in conjunction like the 5500K series, with actinic blue tubes.
10,000K (Upper end of the advanced or special-need frequencies)
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Reason: Incredible amounts of deep-penetrating blue-white light, making them ideal for deeper tanks and simulating deeper reef environments.
Actinic (420 nanometer wave-length or frequency)
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Reason: All corals and most inverts absorb light at the blue-end of the frequency curve far easier than at the red or shorter end of the frequency curve. Actinics are mandatory when using frequencies below 10,000K.
Think of actinic blue tubes as "boosters" to be used in conjunction with the regular 55-6500K range of bulbs. It's the blue light that really makes the difference in photosynthesis, and more is better in an aquarium!
50/50 Lamps
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50/50 Lamp phosphors are 50% 6000°K natural daylight and 50% Actinic 03 blue. These bulbs are an easy way to get the best of both (Daylight & Actinic) worlds in one bulb if you have a single light fixture.
