With the side glass on top of the base glass all it takes is your stand to be a little wonky and the bottom is sure to crack. Even the cheap Hagen tanks are footing bottom when you take the trip off. I have had a couple tank built with not a floating bottom & both have cracked shortly after being set-up. ( I didn't build them ) 
 
If you post exactly the inside dimensions you want I can tell you what sie to get your glass cut & I would suggest to go with 10mm glass it is much nicer to work with and looks great. 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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