I agree with Wes, just add some 1/2 plywood on the inside lip and some thin foam to suck up the little imperfections and voila, you're good to go! 
 
My guess is (and this isn't a hit against your woodwork) is that the lip wasn't perfectly flat along the entire perimeter and that cause some stress across the bottom pane...... *POOF* 
 
Get back in and put it in the same spot, look good there from what I could tell in the pictures. But I also know what your concerns are. I'd **** a brick if my tank were to pop on the hardwood! 
		
	
		
		
		
		
		
	
		
		
	
	
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