Question

How does a fish manage to rise up and go down in water?

Answer 1

In order to rise up, air diffuse from the body of fish and flows into its bladder. As a result, the volume of the fish increases and so the upthrust increases. In order to come down, air diffuses from the bladder and the fish thus empties its bladder. As a result, the volume of the fish decreases and so the upthrust decreases.