Correction from above information: Some SCUBA tanks are only rated for 2000 psi, and can be either steel or aluminum. Other newer tanks are rated to 3000, and a few to 3500 and do not depend on material from which they are made. If the tanks/cylinders are relatively new (within the past 20 years) they are most likely at least 3000 psi rated.
ETA: In order to fill the rifle, you will need a fill device. This will have either a SCUBA yoke for most SCUBA tanks, or a DIN screw in fitting, and a pressure gauge with hose and bleed off valve. Some newer SCUBA tanks and SCBA (not rated for under water use) tanks have threads that the DIN (a German standard which is slowly becoming an SI or International Standard) will fit. At the end of the "whip" there is usually a Foster type female fitting that will fit most devices that plug into the rifle.
If the particular rifle has a proprietary fitting, then it will usually have a male BSPT (British Straight Pipe Thread fitting) on to which adapters can be had that will allow convenient filling.