Avicennia, obtain oxygen by its root which sticks out of the ground. Not only that, Avicennia also has pneumatophores which allows its roots to breathe even when submerged.
The anglerfish has an elongated dorsal spine that supports a light-producing organ also known as a photophore. Through a chemical process known as bioluminescence, this photophore can produce a blue-green light similar to that of a firefly on land. With this light, the anglerfish could see in the deep, dark sea.
Freshwater. Water with dissolved salt content measuring less than 1% is called Freshwater. Freshwater is available from two types of sources, stagnant water bodies, such as, Lakes, Ponds, & Inland Wetlands; and flowing water bodies, such as, Rivers and Streams. Freshwater covers only 1% of the total earth’s surface.
Saltwater. When water has high quantity of salts dissolved in it, it is called Saltwater. Owing to the large amount of dissolved salts and the associated electric charges on these salts, the conductivity of saltwater is high and it tastes salty. Sea, Oceans, Rivers, and Saltwater Ponds are the natural sources of saltwater, which add up together to cover 71% of earth surface.
If the salt water fish were to be put in fresh water then their high salt content inside their bodies (to prevent too much salt diffusing into their bodies) will have it diffusing out down the concentration gradient; killing them. If the fresh water fish were to be put in salt water then their low salt content inside their bodies (to prevent too much salt diffusing out of their bodies) will have it diffusing in again down the concentration gradient; killing them.
They are adapted to salt water conditions. There are many other minerals in salt water besides salt many species of fish need those other minerals in order to survive thus they die in fresh water.