It is a DOT requirement. NO ONE will fill a tank that is out of its 5 year cycle. I had mine done at a scuba shop. Typically the scuba shop is less expensive than taking it to an FBO.lawrence said:according to aerox: my aluminum oxygen tank needs a "hydro test" every 5 years. is every body having this done? any trouble getting your tank refilled if "out of date"?............thanks.....