Floating and sinking

Floating in water

This phenomenon occurs, if the submerged body has exactly the same density as the fluid. The body will remain submerged in the fluid, but it will neither sink or float. The body's volume equals the displaced volume of liquid. Similar to floating on water, the buoyant force equals the gravitational force.

Picture above:
Fish can increase or decrease the volume of their body because of their swim bladder (see picture below) which can be filled with more or less gas (oxygen, azotic, carbon dioxide). By doing this, they can change their density dependent on the current water depth and they can therefore float without wasting energy in swimming.