Exhibit: Swim Bladder
The fish has a small swim bladder made out of a polythene pipette. Polythene doesn’t float -it is the air sealed in the pipette that keeps the fish afloat. The overall density of the fish is slightly less than the density of water so that it just floats.
When you press the button you increase the pressure inside the water tube, This squeezes the polythene pipette, making it a bit smaller, so the overall density of the fish increases (it weighs the same as before but it has been squeezed to a smaller volume). This means the fish will sink if the overall density becomes greater than the density of water.
When you release the pressure, the fish ascends again.