Aqua lung tank

First I would go to fire department and make sure it goes to 3000 psi. If it’s made of steel it should be fine, the aluminum tanks don’t go that high. Then you may need an adapter depending on which gun you get. The Marauder uses this 

https://www.pyramydair.com/s/a/Air_Venturi_Foster_Female_Quick_Disconnect_to_1_8_BSPP_Male_5000_PSI/4926

Aside from that you should be fine the fire department will let you know if the tank needs to be hydro tested or not. Here is a good article

https://www.pyramydair.com/blog/2012/04/compressed-air-tank-capacity/
 
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.
 
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.


BSPT = British Standard Pipe Tapered. I think you meant to say BSPS. As a side note. All BSPS fitting seal at the fitting base with a washer. All tapered thread fittings seal at the thread typically with Teflon tape or a Teflon based sealing compound.
 
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.


BSPT = British Standard Pipe Tapered. I think you meant to say BSPS. As a side note. All BSPS fitting seal at the fitting base with a washer. All tapered thread fittings seal at the thread typically with Teflon tape or a Teflon based sealing compound.

Sorry to be a smart arse but if we are going to be pedantic, it is BSPP designating British Standard Pipe Parallel and yes, BSPT is British Standard Pipe Tapered. While I'm at it. Faber steel scuba tanks come rated up to 300 Bar but I doubt that the OP. has one of these as they are not common in the US.