The maximum size is 12 inches in a 180 gl tank but very few make it to that size, I never said a Niger was an aggressive fish what I said was they are better suited as all Triggers are for a FOWLR, A Naso Tang can achieve a length of 18 inches given the proper conditions, Many are not. A Clown Tang is a very striking fish but should be kept in tha latter as well. I suppose anyone can purchase a 3 inch Trigger or Tang and put it in their 55 Gl tank but keep it long term and have it thrive to their maximum potential is another story. The Blue Throat will attain approx. 8-9 inches much better suited for a Reef,
I've had a Blonde Naso, Niger, in a 150 and got along fine but they weren't in the foot long size. I'm not really into big fish mainly because our little glass boxes can't sustain their growth, My list of inhabitants not to kill in my tank keeps getting longer and longer.