I have a regal blue tang, yellow tang, and a large naso tang in my 180. 
Mostly the rule of thumb is to make sure they are not the same color or shape. 
If you add them all at once u wont find them fighting over territory. 
However a friend of mine has 3 yellow tangs in one of his tanks, so all rules dont always apply. 
		
	
		
		
		
		
		
	
		
		
	
	
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