It isn't how many watts that determine the amount of light you get, but what kind of lights - T8, T5NO, T5HO, PC, etc. T5HO gives you high light, where good CO2 is essential, PC gives you from low to medium light, depending on how good the reflector is, and T8 gives you low light, but only if you use two bulbs. You can use a light that would give you high light if it was sitting on top of the tank, and reduce that to low light by suspending the light fixture a few inches above the top of the tank - about 6 inches for a single bulb T5HO or 2 bulb T5NO light.
i was recommending this light as it does not have individual reflectors and neither its T5HO, i think this regular T5 will give this person medium light if it sits on top of the tank. correct me if am wrong.