Here is an overly simple explanation.
Like houseplants, aquatic plants have different light and food requirements.
Low light plants will usually grow well with 1 to 2 watts of light per gallon (no acitinic lights) and require less fertilization and no CO2
. They tend to grow slower than medium or high light plants.
Med. and high light plants require more than 2 watts of light per gallon, regular fertilization and will probably require C02 injection.
Thats an overly simple way to put it.