I have 3- 300W heaters in my 150 gal sump/77gal DT. 
Redundancy, Redundancy, Redundancy. 
Usually I have only ever seen 2 heaters on at a time, but if 1 ever fails, not that big a deal, 2 heaters go - you're in trouble but not dead... yet.  
When I get my 210 going, I'll probably get another 3 - 300W heaters and have all 6 in there. 
		
	
		
		
		
		
		
	
		
		
	
	
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