This may be safe, but what about nutrient issues with tap water?  How do you know what's going into the system every time you do a water change? 
 
My LFS sells RO/DI for 50cents/gal.  Your water changes would cost $1.25... 
 
If you have an RO system already for top off why not use it for water change as well? 
		
	
		
		
		
		
		
	
		
		
	
	
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