Got a DIY but 6.5-6.7 of the effluent keeps things steady. 
  
You might have it covered already but I was also finding hard to keep the Ca up. Clued into fact that the water I was using for changes was way low on the Ca and Mg. Once started ensuring my change water was up, no problem with reactor maintaining. 
		
	
		
		
		
		
		
	
		
		
	
	
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