The Kelvin intensity that you are referring to “5500k to 10000k” all depends on what corals you want to keep and a bit of personal preference. The real question you should be asking is what watt bulb will I be running over a tank that size, and again the answer to that question lies in the animals you wish to keep in you tank. I don’t want to start a watts per gallon debate or any other type of lighting flame so this is all I will say. “ If you have a coral that needs to be near the water surface and requires the intensity of a 250 watt 6500k bulb, then that is the bulb that you will need, REGARDLESS of the size of your tank, 20g or 90g, that is what that coral will need to survive.” HTH