I used to work in a SCUBA shop, (10 years ago) most SCUBA tanks need to be visually inspected every year and must take a hydrostatic test every 5 years.  (a hydrostatic test involves filling the tank to capacity while in a water tank, in order to tell if the volume of the tank changes as it is filled.  If it does change, the tank is bad.)

another one that also runs on air?