Roberts Oxygen, they generally exchange tanks, check the phone book for your local outlet. I exchange 20#'s for $23 or so.
Maryland Fire Extinguishers, (I think) in Rockville, will fill while you wait. Little more expensive than Roberts is, now, used to be cheaper.
Aquascape? I'm a crypt farmer.
It's a fine line between fishing and standing on the shore looking like an idiot.
That IS an aquascape, it's titled "The Vacant Lot".