There's a limit to how many fish you can put in the tank and keep it healthy. Just because a hippo tang doesn't seem stressed doesn't mean you can put it in a 55G tank. They need the swimming room, and if not accommodated they will become immuno-compromised.
Before you get a fish look into it on say a website like liveaquaria.com, it will give you the minimum tank size required for the fish. If you have a stocking question just come ask us here on AA, there's much more to stocking than inches of fish per x-amount of gallons, regardless of SW