I myself have a very happy 2-3 year old tang in my 66 gallon. He is very happy and healthy and has room to swim. I see no issues with this as he is the only large fish I have.  As long as you care for them and keep them happy and healthy I don't see any issues with a smaller tank like mine keeping a tang.  
 
I wouldn't get any more though. haha 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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