Two of the leading factors when it comes to growth is region and habitat. No two fish even from the same larvae will grow at the exact same rate let alone reach the same exact size. While fish can display excitement and aggression they're simply incapable of being happy or sad. My recommendation is to plan for a six inch specimen and while 2 inches one way or the other can break up a marriage it's not likely to affect your tank. Most of those who use the 6 foot this 180 gallon that lack the knowledge of the original origin of that literature. The options published on the subject were always meant to represent the adequate housing of adult specimens. There's absolutely nothing wrong with raising tangs in smaller aquariums.