I was running my SWC 120 on a twenty five gallon system for a couple of months, and it seemed to work just fine. Everything did well: even my Kenya tree and other softies. I don't know how they would have done long term. 
 
I use it in my 55 gallon system right now, and I really like it. These skimmers do seem overly sensitive to water level changes, so an ato of some sort will certainly help in keeping it functioning well. I just got my ato worked out earlier this week, and the change in the skimmer's performance is noteworthy. 
		
	
		
		
		
		
			
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				Shelley
			 
		
		
		
		
	
		
		
	
	
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