This is my opinion on low light/high light. This assumes normal output flourescents. Low light 1.5 - 2.0 watts/gal. Med light 2.0 - 3 watts/gal. High light over 3 watts/gallon. My definition is not based on growth rates, but on the level of light at which the management of a tank becomes more complicated. My favorite tank so far is 2.0 - 2.5 watts/gallon with co2. You can grow lots of plants, get good growth rates, and have fewer problems with algae. I am amazed at the number of people new to plants who have lights in excess of 4.0 wpg. At that level you better be an expert.

Henry