You can't really use too much other then wasting money and the smell of  vinegar will be stronger through your house/garage... not that vinegar  is really expensive. 
	
	I usually use a gallon if I'm not in a  hurry and the tank is going to sit anyways for multiple days. If you  want want to speed it up... use more vinegar. Not all the salt-creep,  coraline, and hard water stains will come off on their own. Sometimes  after letting it soak in the vinegar for a couple of days you need to go  in there and wipe off whats left with a scrubber pad. But at that point  it should wipe off very easily.