Some species can live in both. Examples:
Striped bass
American shad
Bull shark
Rainbow trout(steelhead)
Salmon
There are fish that are commonly called "Bream" in both fresh and salt water.
There are both marine (salt water) and fresh water fish in the tropics.
Both kinds of fish can live in a delta area. The line between salt water fish and fresh water fish seems to be getting blurrier. Sharks and other typically salt water fish are found many miles up stream in rivers that empty into the ocean. It appears that salt water fish adapt better than fresh water fish as the fish found in fresh water are not found out at sea.
Fresh water
It is not recommended to mix salt water fish and fresh water fish. It is required to separate the two and provide each with the proper tempered water.
Both the Marine Biologist and the Ichthyologist study fresh and saltwater fish.
Most salt water fish will die if placed in fresh water.
Well it depends on the fish. If it is a fresh water fish then it lives in fresh water, salt water fish live in salt water.
Both fresh and salt water, depending on the species
of course is sea water fish conquer more. due to 68 percent of earth is cover in sea water 3 percent is fresh water and glaciers
Both fresh water and salt water occur in nature, so both are natural. Note, however, that fish that live in fresh water generally cannot survive in salt water and fish that live in salt water generally cannot survive in fresh water.
Fresh water.