The balloon displaces the air it's in with its volume of hydrogen, which is.......... lighter than air! (write that down )
The combination would be ... lighter.
The submarine displaces its volume with the equivalent amount of water.
If the aquarium is filled to the top with water, and the submarine is then placed in it, the excess water slops over.. Eureka!
The weight on the scale changes, relative to the mass of the sub relative to the mass of the displaced water.