As said above make sure that the tank is in test. I got mine from a local dive centre with a test on it. If you're not using it for diving then you can get a pressure test that lasts 4 years put on it for £40. It's like an MOT for it as many places will not fill it if it's out of test.
*shameless plug*
I have a 12L certificated tank with gauged fill station, 2x 48CL tanks, Ninja regulator and hose going for sale. All the kit is in excellent order and looks fresh out of the box (except for the 12L tank is not as cosmetically fresh but rock solid where it counts) and a good price. I'm only selling as am getting into my shotguns and the HPA isn't getting used.
Simon.