How old is your rock? 
 
I've had tanks in the past where no matter what I do, some rocks will just grow HA no matter what my phosphate tests show. 
 
Do you have any sea hares? 
 
I'd add a few if not, they make for a terrific biological control and while more of a band aid that a solution, they do work. 
		
	
		
		
		
		
		
	
		
		
	
	
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