Also, keep an eye on your ISO settings. Try and shoot with the number as low as possible. That will keep the graininess of the picture down to a minimum. If the ISO is on auto, then look in your settings to see if you can limit its upper value while in auto. I'd say don't go past ~1600 iso. I don't know your particular camera though so that may be a little low. If you can get everything still enough, shoot at iso 400 all day long. This may work for stationary corals, but will likely not be a good setting for swimming fish. Those will be the ~1600 iso setting. 
 
Good luck! 
		
	
		
		
		
		
		
	
		
		
	
	
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