If all you are going to do is fish, then the only light you need is enough to illuminate the tank to levels that the fishes you are holding are used to.... brighter for shallow reef dwellers, not as bright for deeper reef species. Of course figure in enough light so that you can view them properly as well.
Most folks prefer to have a mix of "white" and "blue" light in their tank. The blue light adds an illusion of depth, like you would see on a nature type program, and is beneficial to most photosnthetic organisms, which in a fish only tank is not of importance. Blue lights can also be used to simulate a dawn and dusk period, so as not to shock your fish should you use a higher wattage, brighter illumination system.
White lights are necessary in most systems to bring "daylight" into your system. If you opt for only blue spectrum lights, you will lose a lot of the color from your inhabitants. While blue does help flouresce your fish, without white you miss out on the brilliance of a lot of species.
That said, if I was to recommend to you a light setup for your tank, one that was only going to support fish, I would probably start with a double ballast 55 watt power compact fixture. If you are able to spend a bit more you could upgrade to 96 watts, assuming your tank is long enough to handle the bulbs. In this configuration, if you would like a brightly lit tank, you could run a 50/50 bulb in one socket, and a "daylight" or "full spectrum reef" type bulb in the other. For a deeper environment, you could go with 2 50/50 bulbs, which will be not as bright, but stronger in the blue tones, making for a very nice display that will not shock the deeper dwellers.
If you are lucky enough to get a balast where each bulb is on a separate circuit, you can set a timer for each one, having the 50/50 come on a half hour before the daylight bulb, and turning off a half hour later, so give your fish a warming up, and slowing down period.
Also, this setup will be ideal, if at some point you want to go reef, because you can add in metal halide bulbs of appropriate wattage depending in your tank depth, and use the PC lights for full blue, giving you the lighting necessary for corals, anemones, and clams etc.
If you want to get ahead, and money is not as big of a problem, you sould invest directly into MH lights, a lot more expensive, but necessary for most non fish inhabitants in the trade. As i dont run MH, others might be able to give you better advice if you decide to go this route from the start. All i know is, the higher the temperature (6700k, 10,000k, 12,00k, 20,000k etc.) the more blue the light output gets, so bulb selection can be very important.
Hope that helps. A lot of lighting is going to depend on if you have a hood already, where you have the freedom to mount whatever you want in them, or if you have to buy a light fixture that is already fabricated. Look for "retro" or "retrofit" kits if you are adding lights to your existing hood by the way.
Kruxy
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