I do not understand what the faulty heater this guy sold you has to do with what temperature you keep your tank at.  That is really none of his business, and I also agree with the above. 
 
Ours was around 75 and we just raised it up to around 80 because that seems to be where most reefers like to keep it. 
		
	
		
		
		
		
			
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28 Gallon Saltwater Reef Aquarium
			 
		
		
		
		
	
		
		
	
	
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