These guys always seem to have the biggest bang for the buck on replacement light bulbs - The 400w 10,000k and 20,000k are about 55 bucks there, 90-125 at many other places.
Difference of a 10,000K bulb and a 20,000k bulb basically boils down to the peak wavelength of light that each will produce. K
(Kelvin) is related to temperature. Basically as the temp. goes up, the further into the Blue spectrum the peak wavelength/color will fall, go higher and you are into the violet spectrum and then thereafter into the Ultraviolet spectrum.
Generally you can get the peak wavelength of the Bulb by this formula:
PwaveLght=2,900,000/Temp in Kelvins so for a 10,000K bulb the peak will be
2,900,000/ 10,000K or 290 nanometers, for 20,000K bulb the peak is at 145 nanometers (short wavelength, higher frequency peak).
a Kelvin relates to the amount of heat that must be applied to a dark object to generate that particular spectral mix of light. For example, a 2000K Light would be if you heated the dark object so that it was redish, 4500K and it begins to get bluish, etc, etc.
Important consideration, generally us aquarium folk go way overboard on lighting. The needs of the animals you are keeping likely falls way bellow the 20,000k spectrum and maybe even way below the 10,000k spectrum (see Doc
Fenner's info on this at wetwebmedia.com). Also, be careful because as these bulbs go up in K
temp they also get dangerously close to the UV
spectrum - i.e. MH
bulbs do output some UV
(not good for you and not good for your Creatures). So High K
temp may not be the best thing all around. Recall that natural sunlight is about 5800K.