PraxAir...if you want the same tank back, like if you actually bought a new tank, you have to carefully explain to them that you want to maintain ownership. They send all the tanks to a filling facility in Tacoma I believe. Typically you just get a swap out tank, but I am a bit skeptical of how other people handle tanks when they have them.
I painted the handle a bright color and zip tied on a brightly painted acrylic disk (yes, you can use those holes you cut out for something!) with my name and city. Because I don't do the swap out thing, I own two tanks. That way I always have one running while the other is being filled. It is about a 2 week turn-around when you do it this way.