For those that frequent the north side of the beltway, Harford Beverage at Harford and Taylor Rds. (just inside the beltway) does tank swaps up to 20 lbs! They were $12.29 for a 5lb swap. The $10 savings over Roberts Oxygen makes for the perfect reason to grab a sixer of your favorite brew! They have a pretty fantastic, if a little small, selection of beer and wine.