So the assumption is that you do not have an overflow going to a sump that would constantly remove the top layer of grime from the tank. 
 
The following has worked for me. 
 
1)  Pointing a powerhead or return at the surface of the tank to keep it clear.  You may need a couple on a 135. However, this seems to just relocate the grime to the corners of your tank. 
 
 
 
2) On my refugium, I have a suction tube that that just takes the top portion of my water and send it to my sump.  But as I said before, if you do not have a sump, this will not work for you. 
 
Chris 
		
	
		
		
		
		
		
	
		
		
	
	
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