I was having similiar problems in one of my nanos and put in a 100ml pouch of Seachem Purigen & it solves the problem.  
  
I replaced the carbon in my HOB with it and things look great.  Water is crystal clear, and my corals (no SPS) are more extended. 
  
Andrew 
		
	
		
		
		
		
		
	
		
		
	
	
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