Usually the ability to grow plants really relies on how much actual light reaches the plants. For example, if you have a ball of java moss on the floor of an aquarium with a light bulb 40w, that is 12" tall tank, it's going to get more light and grow better than a ball of java moss on the bottom of a tank that is 36" tall with the same lighting.
You don't want to over-light a tank because then you may end up with deficiencies and need to fertilize, which may then require you to use C02. For me, personally, I like low-tech setups. a 10 gallon with 1 20 watt fluorescent tube has always been the perfect amount of light for me, no c02, no ferts needed, and still results in beautiul, lush green plants that don't grow out of control.
What dimensions is the tank?