I went to a dinner party at the Monterey Bay Aquarium. They set the tables up in front of the massive wall which holds back 1,000,000 gallons. Now that's a large tank....
BTW, this tank has a school of yellowfin tuna running around in it. There was also an ocean sunfish (mola mola I think)which weighed in at 600lbs when I was there.
I'm not sure exactly how much is in there, but the oceanarium display at Chicagos John G. Shedd aquarium has somewhere between 1.5-2 million gallons. Holds dolphins and a couple of Beluga whales.