What to do:
Move the plunger up or down.
What happens to the fish?

Think about it:
Many fish control their buoyancy through gas filled bladders. Enlarging the bladder by adding gas enlarges the whole fish slightly and so increases the amount of water pushed out of the way. This increases the buoyancy and makes the fish rise.

Some fish which live at a shallow depth can increase the gas content of their swim bladders by gulping air at the surface. Other fish that live at a greater depth have a gas gland to produce gas and a resorptive area to remove it.

Whether a fish floats or sinks depends on two forces. The fish is pushed upwards by a force equal to the weight of water it displaces. This is the force which causes buoyancy. The second force is gravity which pulls the fish downwards.

If the buoyancy force is greater than the force of gravity, the fish rises. If the buoyancy force is less, the fish sinks. If the forces are equal, the fish remains at the same level.

Submarines can control the amount of air and/or water in ballast tanks to control their bouyancy in the ocean.

The human body only just floats, because the force of gravity and the buoyancy force are almost the same. To increase your buoyancy force, you can wear a life jacket. A life jacket becomes part of your body without adding much to your weight. In the water, your life jacket displaces a lot of water to increase the upward buoyancy force. The life jacket helps you to float higher out of the water.