Well let me confuse you a bit more. What you are really looking for is PAR. The K
rating refers to the color temperature of the light the bulb produces. For example, a 20,000K bulb is very blue in color where a 10,000K is very white. The watts per gallon rule is somewhat outdated. A watt of PC
lighting is not equal to a watt of Metal Halide lighting so the rule is flawed. Typically the higher the K
rating of a bulb the less PAR it gives you. Look at PAR as the intensity of the bulb and how well it penetrates the water. Lower the K
number = higher PAR value.
The amount of lighting for your tank is dependent on what you want to keep. Corals, clams and anemones require more light then a fish only tank.