Try looking at your local home depot for GE Window and Door silicone.  You want Silicone I, not Silicone II.  The stuff is a fraction of the cost of "aquarium" silicone sold in pet stores.  I used it to put together the internal overflow box on my 30 gallon... all the inhabitants in my tank have not shown any signs of poisioning... (granted there's only a xenia, a couple of mushrooms and pods in my tank right now). 
 
-Richer 
		
	
		
		
		
		
		
	
		
		
	
	
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