In reef tanks any thing over 18 inches is considerd deep. At a depth over 18" the 250 watt halides are recommended with 150 watt for depths above a8 inches. That is to keep SPS anywhere in tank. But as already written above, softies, leathers and some large polyp corals can grow fine with less light. It is much easier to find spots of lesser lights for softies, leathers and LPS corals than it is to try to get many strongly lit spots for light loving coral in a less brightly lit tank. Not many people like the look of a tank with all the corals stuck up in the top half or third of a tank. Ther are commercial propagators regularly using 400 watt halide lights on tanks only 24 to 36 inches deep.