The tank would stay submerged because it was filled with.....water? Just like you don't need to tie down the tank on your table the water would more than hold down the tank. Displacement also would be extremely minimal because its just the glass/acrylic that is displacing the water.
My understanding of the OP's post was to put a smaller tank inside a larger tank, allowing the water to travel between the two, but having some type of closed top with holes small enough for fry/eggs to travel back and forth but not allow the larger inhabitants to get inside. This is a very unique idea IMO
, and would very cool if done with contrasting colors (say PFS in the small tank and black substrate in the main tank).
It's basically just a breeder net/tank that doesn't float...