welding gas tank

[mbarre@JUNO.COM (Matt Barre)]

Here's an off the wall thought.  I recenty purchased a Century mig welder
and was going to buy a gas tank from Harbour Freight when I spied an old
steel SCUBA tank left over from when I used to dive.  It is out of
hydro, but would it be possible to change the fitting and use it for
CO2???   Would the normal gas suppliers not want to fill it?  I would
paint it green and stencil it CO2 so no one would try and take it for a
swim!
TIA.

Matt Barre
Mobile, AL