With your tank being 2' deep I'd think you'll need an 8 bulb fixture, plus the width of 2' will be hard to cover with a 6 bulb unit 
I use an 8 bulb unit on a tank 19 tall and 18 deep 
  
I love my 8 bulb T5 and wouldn't trade it for anything else currently available, unless I were to fall into possesion of some dimmable ballasts 
  
I only change my bulbs every 12 months, not 6, or 9, and I spread out the changes throughout the year to not only offset the costs but to not shock the tank 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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