If the system was started with all parameters in check then C should equal B however life is never that simple, so in the beginning of dosing while you are balancing you will get a difference, once in balance you "should be able" to get C pretty close to the same dosing as B. If you notice the Mag is out of balance then add Mg as a different element, but this should not be required. 
		
	
		
		
		
		
		
	
		
			
			
			
			
			
			
			
			
			
				
			
			
		 
		
	
	
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