FedEx. Depending on the size of your tank, you may have to use a shipping company, or freight shipping. Very large tanks come on pallets and are delivered through freight/shipping companies.
Or you could do what I have in the past. Check out uship.com. I have never had a problem with anything I have shipped. If you crate it a broker will probably find you something cheaper than fed ex on there. For example the last iem I shipped was a 46" printer for here to Arizona for 260 and some change.