You could use a pair of FishNeedIt or Coralife 2 bulb T5HO lights, with one near the back and one near the front of the tank. That would give pretty uniform light, and still be low light so you wouldn't need to use CO2. A single light fixture, even with 4 bulbs, will not cover the whole 18 inches of front to back width uniformly unless it is raised several inches above the top of the tank. Adding window screen to reduce the intensity will work, but you would still lack uniform light that way. The reason for using the Coralife or FishNeedIt lights is that they are relatively inefficient, so they don't give you as much intensity as most other lights do.