Fish can adjust their buoyancy. They change their volume without changing their mass by inflating or deflating a balloon-like internal organ called a swim bladder with gas from chemical reactions.

In an estuary (the boundary between a fresh-water river and the salt-water ocean) the density of water in a particular region can change as the tide goes in and out and the mix between the less dense fresh water and the more dense sea water changes. If a fish of mass 3000 g can float just above the bottom of a volume of estuarine water (density ρ = 1.05 g/cm³), by how much does it need to change its volume to stay where it is if the density of the water changes to ρ = 1.060 g/cm³? Explain your reasoning.