I personally would go for 30" - 36" width. 72"x24" gives you the same surface area as a standard 180 and won't give you much more space for corals and fish than a standard 180 would. Also, with the 6'x2' footprint, your loking at 30" tall to get 225g. For 270g, your looking at 3 feet tall. For 300, your looking at 40" tall. If you were to go with a 36" width, you would need 30" in height to achieve 336g, or only 24" tall to achive 270g.