A fifteen gallon aquarium housing 5 fish and other creatures means that you are overstocked, I think.  I have five fish in a 29 gallon and I consider myself to be right at the limit.  As others have said, the tangs are going to need a new home regardless. 
		
	
		
		
		
		
			
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28 Gallon Saltwater Reef Aquarium
			 
		
		
		
		
	
		
		
	
	
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