“This tank looks suspiciously like an oxygen tank, but there is a small skull and crossbones symbol etched in the bottom. If you hadn't examined it closely, you wouldn't have seen the symbol.”
The poison tank is a quest item in Fallout 2.
This tank is specifically built to help in sneakily assassinating Louis Salvatore, thus earning a reserved spot in the family Mordino. Other than that, this tank has no use whatsoever.
Hidden in the basement of the Desperado casino, in a room full of blue crates and rats. There is a crowbar in the same room.