How much light is required??????

[Delphinus]

If it helps, I don't think there's such a thing as "too much light" but the caveat is that there is such a thing as "too much light too fast." If something is coming from a darker tank it could suffer light shock if not properly acclimated to the higher intensities, but usually things will adapt and be fine. I had 2x250W supp'd with 2x110W over my 18" tall 75g for 7 years before I shut that tank down last year, the halides were maybe 4" over the surface.

The nice thing about an 18" tank is that it's not terribly deep so it's a bit of good fortune in that you can pick pretty much any lights you want (t5, halides, etc.) and I think it would be hard to go wrong with any of them.