A typical SCUBA cylinder holds 3000 psi of air (not oxygen, at least not usually more than 50% and that's very rare) Assuming the cylinder was never allowed to go completely empty and suck in water and the visual and hydro are current, the air in the tank will be well filtered and shouldn't cause problems.
To do this you would need a primary SCUBA regulator to drop the 3000psi down to something that won't destroy everything around it. A primary reg will drop that down to 90-150 psi. From there, you would need a secondary reg (like a CO2 reg, not a secondary SCUBA reg) to drop that pressure down to a workable 0-30psi.
IMHO you would be better off selling one and getting an o2 tank or just an aquarium pump and filter. The compressed air in a SCUBA cylinder is no different than the air you're breathing right now which is basically 21% O2 and 79% N2. So other than having ~ 80cuft of clean dry air in a bottle, you will not be able to oxygenate any faster than an aquarium pump setup.
Having said that, you can get cylinders that have been "oxygen cleaned" and have higher concentrations of O2 in them, but as mentioned above it's almost never more than 50%, more commonly 28% or 32% but it can be whatever the diver specified. This is called Enriched Air Nitrox, EANx or Nitrox and the cylinder will be clearly marked with a big band that it is a EANx cylinder.
Bottom line is unless you have the primary regulator and a spare secondary regulator laying around, coupled with the fact that no reputable SCUBA shop will refill it if you're not certified and getting EANx fills is a separate cert; it's probably not worth it.
Derrin
