If all you have room for is the 30 gal that would not be too bad. I actually like the footprint of that one better - more front to back room to aquascape. What are your intentions for this tank? A reef needs extremely stable conditions which a larger tank helps to control, but also needs a lot of light, and a shorter tank allows for better penetration. In all reality, you can not really get any cooler fish for a 55, and you would probably only be able to get away with 2-3 more fish. I actually downsized from a 72 to a 40 and love my current tank. I do wish I had a larger tank (in the 125 gal range) but my 40 is great.