The plywood thing is only for the floor itself, not the joists.  If you have "rails" on the bottom of your stand, they will spread the load out over the joists.  Plywood really does not carry any load when laid flat.  It's strength is on edge. 
		
	
		
		
		
		
			
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				Dan Pesonen 
 
 
Umm,  a tank or 5
			 
		
		
		
		
	
		
		
	
	
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