Because saltwater fish can only breathe in saltwater (as in the ocean) and freshwater fish can only breathe in freshwater (as in tap water.)
There are freshwater drum, and saltwater drum species.
saltwater
They are freshwater fish.
A saltwater fish tank can be half freshwater and all of the saltwater fish will live but not for very long.
saltwater
They are freshwater fish.
Certain freshwater fish can survive saltwater conditions; examples are black mollies, mono's and scats. All of these fish aren't truly freshwater or saltwater, they are Brackish. Brackish is between freshwater and saltwater, and so these fish are highly adaptable.
They are Saltwater fish.
Well, there are 2 types of fish, saltwater fish, and freshwater fish. Only put in saltwater if you own a saltwater fish. Note: Saltwater fish could live safely in freshwater, but freshwater fish will suffocate in saltwater.
Redfish are fresh water
No, freshwater fish must be kept in freshwater and saltwater fish must be kept in saltwater. The one exception (that I know of) in nature is salmon which migrate from where they were born (freshwater) to the ocean and then back to where they were born again to breed.