It all depends on the fish you wanna stock your tank with. Some are schooling fish and prefer to be in schools, some dont venture far from where they make their home, some like to swim in open water a lot like tangs. If you want more then one tang, you need a lot of room so they will have enough space for all to swim and enough live rock for them to graze on so there isnt a lot of aggression over grazing grounds.
Different fish need different things. Thats why you research what you would like to put in there before you buy it, and before you make the actual purchase, research some more. A lot of newbs dont research what they buy, Just because you read it at one place, doesnt mean its the only way, get multiple sources and make sure they are not quoting the same person. Find somebody who is doing something similar to what you wanna do, and ask them questions and the ask them some more. Most people in this like to brag, so they'll answer your questions, lol. | 
