My guess would be 45 to maybe 50 LBS for a 33 pound AL tank filled. Really think about mounting it on the side of the machine. The beauty of the compact loaders is their ability to get into tight places. You would be adding at least a foot to the with. Its not that bad on the foot platform. Im putting a fold down foot peg behind the tank. Like the ones on a Van or box truck that way I can move around without being cramped.
Also the tank hanging off the back is added counter weight. Todays job the trees (Eastern Hornbeam) took 2 guys standing on my Ramrod to counter weight them.