I agree with Albert, if I were doing a 250 on a 36" tank I would try and do two. Having said that I think that a single 400w would work pretty well too, but the overall coverage still wouldn't be as good as 2 x 250's. The aquascaping of the tank will play a big part in this as well. If it's a very open aquascape with the majority of the corals to the center of the tank and not stretched out to the end panels, then a single 250 could be more than plenty for you.
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