Warning this may be overly complicated but I wanted to add some of what I consider to be important information about lighting and kelvin ratings.
First off I am not disputing what Oldman wrote about the 18000K vs. 8000K as this is more often than not the case but biology hates rules.
If your not bored yet...
I would like to add to this that the kelvin rating of your bulb is not the whole picture in terms of the value of lighting for your plants. Most plants use light in both the red and blue spectrum for photosynthesis but do not use light in the green spectrum. Chlorophyll reflects green light absorbing best in the blue and has a secondary spike in the orange/red. This is why plants generally look green. In water the red wavelength is lost very quickly and many corals grow beneath the leval at which red light is plentiful, which is why most marine lights tend towards blue. In freshwater systems most plants grow high enough in the water column that they still recieve a decent amount of both of these. Many freshwater plants use secondary photoactive pigments that spike in the reds and orange wavelengths phycoerythrin and phycocyanin. Plants that use these pigments in large amounts will be those that look purplish or very dark green.
Here is a link to an image with the light distribution.
18000k aqua-glo image by alexmaoli on Photobucket
The hagen agua-glo 18000K bulb you initially had has a large spike in the orange/red and a moderate spike in the blue with very little output in the yellow and green wavelengths. The 8000K bulbs tend to have similar profiles as your 18000K aqua-glo but the main spike is in the blue and the secondary spike is in the orange/red. So this bulb is actually appropriate for many freshwater plants.
I hope this was at least a little bit useful and not too boring.
