20 pound CO2 tank across international boundry?

Don't try to conceal it and you won't have a problem. I've never seen compressor tanks, compressed air tanks, etc even mentioned in the regs about travel. If they had a problem with a CO2 tank, they would have problems with propane tanks too. It isn't a weapon and you won't even need to declare it. I know I've seen the tanks commonly used with the off-roaders in Canada so doubt it is regulated any more than here in the states.