Looks like tip burn to me.  Where are you pouring the buffer into the tank? Ideally you would dose it in a "high" flow area of the sump (if you have one). If dosing in the tank, dose very slowly in front of a powerhead so it gives it a chance to mix before it hits the corals. 
  
How long have you been dosing the alk buffer? 
		
	
		
		
		
		
			
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