I'm not sure I understand what 24 has to do with your quesiton. But in general, one NO 40 watt bulb will not be enough light to keep any signifigant corals in a reef tank.
Perhaps a mushroom right up top or something.
What kind of corals were you planning to keep? There is a wide range of lighting availble.
I started with 110watt of PC light on 20gallon tank, and have since upgraded to 250w mh on the same tank.
