01-19-2011, 07:19 PM
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Kelvin is the measurement unit for the colour temperature of light [nothing to do with "hot/cold" type temperature]. Light is comprised of various colours, each with its specific wavelength. When you see the "rainbow" you are seeing white light separated into its various colours caused by sunlight refracted through water droplets. Same as a prism.
Sunlight at mid-day is approximately 5600K. A lower K number means more orange/red ("warmer" light) and a higher K means more blue/purple ("cooler" white), in very general terms. Aquatic plants grow best in light between 6000K and 7000K, so one of the "daylight" type tubes having a kelvin of 6500K is perfect. The red and blue is there (aquatic plants need red and blue to photosynthesize) with a balance of green for a natural rendition of fish and plant colours.
"Watts" is the measurement of energy used by the tube or bulb to produce the light. It also has nothing to do with intensity of the light, although in very general terms the higher the wattage the more light is being produced. But with todays better-made T8 (and now T5) tubes and Compact fluorescent bulbs, the energy used (watts) is far less to produce fairly intense light compared to older tubes and incandescent bulbs. They also produce less heat, which is because the energy is not being used so much for heat with the newer types.
Fluorescent tubes are generally the same wattage for the length of tube, e.g., a 48-inch tube is 40 watts. However, the way the tube is made can have a significant difference in the light intensity emitted, and can also use less energy to do so, so the watts can decrease. Some manufacturers make 32w tubes that are equal in light intensity to 40w tubes, other factors (such as Kelvin and lumens) being the same. So when buying fluorescent tubes for a planted aquarium, within the length that will fit your fixture you want to look at the kelvin primarily. Lumens I tend to ignore as most of the daylight 6500K tubes are comparable, at least in my experience. The so-called "aquarium" and "flora" types are not, however, they are usually about 1/2 or less the intensity of daylight or full spectrum.
So, jbonez was correct, a tube with 6500K rating in a "daylight" such as those made by GE, Phillips or Sylvania will work fine; I hope my explanation above helps you to understand why.