Paintballers use co2 like its water, works better than just compressed air. 20 pound tanks are very much explainable, even two or three, and welding/medical supply stores exchange thousands of these a year. If you are worried about it, just know a thing or two about paintballing, maybe a local hot spot, and they wont even give a shit if you are asked. Just don't bring back your tanks for an exchange with resin on them, or crushed foliage on the bottom. And don't buy new shiny aluminum tanks, buy old shitty aluminum tanks, unless you want to He-Man steel tanks around. They wont be yours after the first exchange anyway. Paint them if you like, it's like changing the curtains in the living room. Or put some bullshit team logo for your imaginary paintball team. Or just don't be a pussy and just drop the thing off.