Don't try to get the amount of light you need by looking at watts per gallon. That will most likely just mislead you. Instead, think in terms of the type of lighting, T5HO, T5NO, T8, PC, etc. Your tank is 48 x 18 x 24 (I assume), so if your lights sit on the top of the tank, they will be about 22-23 inches from the substrate. At that distance a single T5HO bulb should give you low medium light - around 50 micromols of PAR. But, the 18 inch depth of the tank precludes using just one bulb and getting uniform lighting over the substrate. One way around that is to take advantage of Catalina Aquarium's cheap customizing prices, and have them make a 2 bulb, T5HO light, using the housing normally used for a 6 bulb fixture, and having the two bulbs separated as far as they can get them - about 8 inches or so. That should give you about the same 50 micromols of PAR, but much more uniformly over the substrate.