Welcome to AC! Kelvin is a measurement of the color of the light a bulb puts out. If you are looking for how powerful a light you would need, you're looking more at either watts, or PAR. If you are doing a malawi tank, with no plants, then generally a standard fluorescent fixture would work. A bulb with a lower K rating (around 3500) will be more yellow, a 6500K would be closer to daylight, and 10,000K would be very white. It would really depend on what looks best to you.