Also, there is the possible loss of Oxygen exchange with a top.  More prevalent in SW tanks than FW as SW doesn't hold oxygen as well as FW.    
 
That said,  there are a lot of successfull covered tanks.  Plexi is probiballly going to be the cheapest and clearest.  I had 4mm glass covering one of my tanks once, and was forever cleaning it.  You could see the light difference even with clean glass.  Now,  I just live with the evaporation.  try keep tank a touch cooler?  room a little warmer.  That will lessen evap too 
		
	
		
		
		
		
			
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				Dan Pesonen 
 
 
Umm,  a tank or 5
			 
		
		
		
		
	
		
		
	
	
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