If I'm correct, none of these fish are fully grown, if they were, they all would'nt be in there, when I feel their isn't enough room, I will upgrade to a larger tank. In this hobby their are a lot of if's and when's. To an extent you have to be prepared, but your reasoning is illogical because when you compare our tank's and territory of course their is not enough room, matter of fact, some of your tangs occupy up to 50M of territory in the wild, so I could say they do not have enough space and that your tank is too small, however this is not true.