From what I have heard, in my opinion the gallons arent as much of an issue. The issue is the length of the actual tank. Most tanks 55-90 gallon's are only 4 feet long. I have been told tangs require more swimming room, so wouldn't a 6ft tank be the best in general? Disregarding the gallons? I mean of course you cant go with a 6ft 55 gallon tank.