James, the number followed by "K refers to the intensity of the light, the "K" is Kelvin. So a tube rated at 11,000 Kelvin will be more intense light than one with 6500K. The higher K is good for deeper tanks as it allows the light to penetrate the water better. As light penetrates water its intensity is greatly reduced, much more than in air.
The general guide for planted aquaria is to have 1-2 watts of full spectrum fluorescent light for minimal plant growth, and 3-4 watts for increased plant growth. Some plants are termed "low" or "moiderate" light and they will do fine at 1-2 watts per gallon; others are "high" light and will not fare well unless provided with 4 watts or more.
But providing more light is not the only thing to consider. Plants also require nutrients, which includes CO2 (carbon dioxide) and micro-nutrients (sometimes called trace elements) and macro-nutrients. All of these have to be in the correct balance with the light in order to have plants photosynthesis which is how they grow. In most aquaria, the fish population provides much of the CO2 and some of the nutrients through biological processes, and the aquarist can add a comprehensive liquid fertilizer to ensure the correct balance of macro and micro-nutrients. With 1-2 watts of light, plants like swords, crypts, Anubias
, Java Fern
, Java Moss
, vallisneria, etc. will grow quite well. At higher light, they will grow faster, and other plants requiring more light will also grow, but the CO2 (in particular) and micro- and micro-nutrients have to be increased to balance the light. If not, algae takes over.
You have to decide what type of tank you want; what type of plants, how much maintainance each week (faster-growing plants require regular pruning), and this will allow you to decide how much light and if CO2 is required.
The figures of watts per gallon works for regular fluorescent full spectrum tubes. The newer compact lights have lower wattages for the same power, and others on this forum have written about this and have more experience. I still use the standard fixtures and tubes. You can see the result in my aquarium photos. I have 1 watt per gallon, no CO2, and twice weekly liquid fertilization.