It is great that you love tangs as I do but you really need to carefully consider providing them with the optimum conditions. You do don't you? 
So plan to move up a couple of feet to a 6 footer and add tangs to your hearts content. 
 
IME it would be poor judgement to add a purple tang which is known to be aggressive to a small tank that already has two tangs in it as has already been posted. 
		
	
		
		
		
		
		
	
		
			
			
			
			
			
			
			
			
			
				
			
			
		 
		
	
	
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