Sorry about this, I know the brain teaser style posts have about worn thin the topic of flies in a jar and fluid/gas buoyancy, thrust etc., but this is a very specific question I didn't get a good answer for while reading the other puzzles.

I can believe that a bird or insect in a jar, or a R/C helicopter hovering within a closed container would cause the weight of the container to be the same whether the heli/birds were landed or flying.

I can easily see how a fishtank filled with water would weigh 1 pound more when a 1 pound model boat or submarine was placed in it.

However, its hard to believe that if you take a hydrogen balloon with a 1 gram weight attached and trimmed for neutral buoyancy, placed it in the container, that the container would weigh more. How is the balloon placing any thrust on the container? Is it exactly the same as a submarine in the fishtank?

My question is specifically relating to the balloon, not birds or anything. Only a balloon hovering in the air, not touching anything.