[Dominic]

You don't actually NEED to put anything on it - but it's considered a good idea. A kid died in America (IIRC) a year or two ago when he borrowed dad's scuba gear and jumped in the pool, whereupon he passed out and died as he'd been breathing argon the whole time..

Go ask your local argon supplier what they want to see in an argon cylinder when you bring it to fill - that'll get you the best answer.

There's no reason why not to use a 3l pony for it, other than that it's a tad large, and that people will assume it's a pony filled with breathing air...