Hmmm! Kinda thinking the same thing here. I started with a 72 gal and the summer heat never affected it in the slightest. No sump either. That is a lot of fish in there. Not sure how big your sump is but I am leaning towards it being due to stress like lee9 said. 
		
	
		
		
		
		
		
	
		
		
	
	
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