Saltwater fish constantly drink water but still excrete concentrated urine to compensate for the water loss. they also have gills to excrete excess salts. conversely, freshwater fish absorb salt from their surroundings using their gills and their bodies reabsorb salt from their urine. this describes how salt- and freshwater fish
a.
use their gills to drink
b.
keep the salinity levels even in their aquatic environments
c.
have adapted to deal with osmosis
d.
have evolved behavior patterns to match their physiology

They do not influence the salinity level in their environment, they maintain the salinity in their body. If they don't, osmosis will cause their cells to shrink due to the fact that the salinity in the cell is lower than outside the cell. This will make water leave the cells. So they need to get rid of excess salt all the time. Freshwater fish need to retain the salt in their bodies, otherwise the opposite will happen. Salinity level in the cell is higher than outside the cell and this causes influx of water which will make the cell build up turgor pressure. Ultimately they will explode. So fishes in both environments adapted to deal with osmosis.