welcome...and i would recommend the same thing skip did...a 20" fixture. the bulbs are only just under 18" in length so it would work great. also, if you wanted more flexibilty in what you could keep...ie light-demanding corals...you could use a metal halide pendant or easily create a custom MH canopy with a 70 watt double-ended metal halide fixture.
let use know which direction you are leaning towards with your coral selection and we can help you out a little more.
jesse