Freshwater fish regulate blood volume and
salts by actively uptaking the salt with the gills of the fish from
the environment with the help of mitochondria-rich cells and then
the water will get diffused into the fish by excreting a hypotonic
urine.
Saltwater fish regulate blood volume and salts
as it has an internal osmotic concentration which is lower than
that of the surrounding saltwater and so, it loses water and gains
salt and excretes salt actively out from its gills.
This process of regulating blood volume and salts is termed as
'osmoregulation'