I too am looking into planting Glosso and have been surfing the net for recommendations. Everything I read indicates that Co2 and lighting are important factors in achieving a carpet like appearance.
Phoenix44 clarified the lighting issue for me - the following is his explanation...
WPG - watts per gallon.
Watts is not a measure of light output, but is the unit of electrical consumption. basically with the technology we have now, a 12W energy saver can produce light that is equivalent to 60W of the old fashioned light bulbs. see where I am coming from now?
so while - per watt the energy saver, produces less light per gallon under the WPG rule, in reality the 60W and the 12W produce the same amount of light.
then there are metal halides which are inefficent, because a lot of the energy they consume is released as heat.
With tubes, the T8's are now being replaced with the newer T5 technology, that produces more light for the same wattage. then there are T5 High output lights that give out even more light for the same number of watts.
So the Watts per gallon / litre rule is fundamentally incorrect, as the underlying scale of measurement is inaccurate and not constant; and thus wrong.
HTH