Another possibility is Brooklynella. Even though it's known as Clownfish disease, it affects all. Some it kills and some are only carriers. 
Brook kills just as fast as Velvet. 
 
I bought some fish last year and the 4 of 10 that survived through a QT period of 3 weeks infected my DT fish. 1/2 my DT fish died within 3 days, including 2 of the 4 that were introduced. 
I had to remove and treat the survivors with CP. 
The carrier fish showed no signs so it was impossible to know it would happen. 
It was my own fault as I panicked when the QT fish were dying and I thought they needed more room than my QT provided. 
 
If you ever need to house fish again during a move, I'd suggest finding someone with a spare tank who can house them completely independent for you. 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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